Refnx

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- bugfix: remove printing of auxiliary parameters in Objective.setp. A remnant
print function was in that method. However, those parameters aren't present
in a global objective, which caused an AttributeError.

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- Added polarisation reduction (Oliver Paull)
- Introduce Objective.auxiliary_params. These are extra parameters that are
modified during the calculation of Objective statistics, and are varied
during a fit, but may not be directly part of Model calculating a signal.
Their main purpose is to aid in creating constraints.
- Add example to show how to co-refine non-spinflip NR data.
- Silenced warning during test from slow reflectivity calculation, which
erroneously gave the impression that the C based kernel wasn't available.

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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.