Refnx

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- check that refnx/analysis doesn't get in the way of handling
multidimensional data.
- add a header to datafiles during reduction.
- make requirements.txt available in the refnx GUI.
- add units attribute to Parameter. This is displayed as a tooltip in the GUI.
- GUI and refnx command line entry point now reads sys.argv to see if GUI
should open an experiment file.
- Associate the mtft file extension on macOS with the GUI app, meaning you can
double click on a mtft file and it'll open with the app. The mtft files also
display with the GUI icon.
- BUG: an error was experienced whilst reprocessing an existing chain in the
refnx GUI.
- BUG: fix gh336, remember the dq/q choice on reloading an experiment file.
Previously the GUI reverted to constant dq/q on a reload, even if it was
deselected.

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- Make default Spline.dz.bounds.lb > 0, so that crashes aren't experienced
when a dz is set to 0. GH549
- Add the shgo keyword parameters 'n' and 'iters'.
- Add max_delta_z to sld_profile so that the point density in an SLD plot
can be changed.
- Don't use tqdm progress bar when fitting in GUI if sys.stderr is None.

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- MCMC sampling in GUI now plots parameter value vs step number, so you can
see how many samples to burn
- Fixed GUI crash when trying to make a snapshot with a name that has already
been used.
- Give BaseObjective a default weighted attribute, enabling it to be used in
GlobalObjective.
- Fallback to numerical estimation of Hessian/Covariance matrix if an
Objective cannot calculate a residuals array (e.g. mixed use of
BaseObjective/Objective in a GlobalObjective).
- Vendor scipy.stats.qmc.
- Use Latin Hyper Cube sampling to initialise MCMC walkers when initialising
with prior. This ensures a good distribution of walkers and no clustering.

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- SPZ batch reduction
- remove some roadblocks to JAXification (float casting in Parameter.value
setter)
- volume fraction of solvent can now be used when constructing a Slab from a
Scatterer.
- Fully remove traces from graph when opening new experiments.
- Store reduction_options when processing spectra.
- RuntimeError if a log transform creates non-finite numbers.
- ReflectModels can now use a q_offset to correct for possible angular
misalignments.
- `possibly_create_parameter` and `is_parameter` now use `BaseParameter` for
their operation, rather than `Parameter`. Thus _BinaryOp and _UnaryOp can be
given to these operations and work. This enables construction of various
objects (SLD/Slab/etc) from constrained parameters, rather than setting a
constraint afterwards.
- MixedSlab is now allowed to be the first/last Component in a Structure.

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- Drop Python 3.6 for new development (as per
https://numpy.org/neps/nep-0029-deprecation_policy.html)
- Pin setuptools version on RTD to allow the docs to build.
- Add a tutorial to the docs on model selection.
- Update emcee vendored code.
- Fix repr of Model
- Fix several warnings during test run.
- Add error bar plotting to refnx GUI.
- Improved the specular ridge finding methodology for data reduction.

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- build more Linux wheels.
- vendored ptemcee (bleeding edge), so it's no longer necessary to install it.
- use -funsafe-math-optimizations for reflectivity calculation.
- fixed bug when retrieving log-probabilities from PTMCMC sampling.
- fixed encoding call for Py3 (.decode doesn't exist for Py3).
- improve pymc3 model creation. pymc3 offers different ways to carry out MCMC
sampling. This may lead to pymc3 being included as a sampling method in
Curvefitter.
- implement `invcdf` method for Bounds objects. This will allow creation of
prior transforms from [0, 1) to the original range specification.
- BREAKING CHANGE: `objective.model.logp` and `Objective.logp_extra` are now
included in the log-likelihood (`Objective.logl`) instead of log-prior
(`Objective.logp`). This makes it easier to work with pymc3 and dynesty.
- Add `Objective.prior_transform` that converts random variates in a unit
hypercube to parameter values, according to their prior distributions.
- manual_beam_find can be sent a name to be displayed on the window title.
- ReductionOptions dict is now used for specifying options for reduction and
processing of reflectometry data. This will permit different settings to be
used for datasets measured in different configurations.
- Add AutoReducer, an object that watches a directory for modified/created
files and automatically reduces them.