Refnx

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- Build standalone GUI as part of continuous integration runs.
- Fix couple of GUI warnings when started from a terminal.
- pyinstaller plist settings adjusted to make retina compatible and include
refnx version number.
- use Github Actions to test and build Linux wheels across all the Pythons.
- Compensate for dq_type being added to ReflectModel (older .mtft files
wouldn't load into GUI).
- Refactor test_reflect to challenge all reflectivity calculation backends.
- refnx.reflect.available_backends lists all reflectivity calculation backends.
- Add pytest fixture to download test data. This dramatically reduces the
size of the package.
- Compensate for _stderr being added to Parameter (older .mtft files wouldn't
load properly into GUI).
- Fixed progress bar for parallel tempered sampling.
- Make sdist

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This will be a bug fix release over 0.1.13. The Linux wheels weren't tested
enough and the default reflectivity calculation backend didn't work correctly
in 0.1.13.
- fix default backend calculation.

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- synthesising of datasets (Data1D.synthesise) can now be repeatable by
providing a seed.
- use Github Actions to test and build macOS wheels across all the Pythons.
- The _creflect.abeles reflectivity calculation can release the GIL for a
large part of its calculation now. This will enable parallelisation using
either Processes or Threads. Processes still have the edge on speed at the
moment.
- change reduction code for ReflectNexus.phase_angle to use degrees, not
radians. (previously angles were returned in a mix of degrees/radians)
- Speed of reflectivity calculation is improved between 10 and 20 %. This is
achieved by the use of C99 complex arithmetic instead of C++ std::complex.
This improvement does not apply to Windows because it doesn't have a
C99 standard conformant compiler.
- Document how to save a model.
- macOS wheels (and CI testing) have the cyreflect openmp option activated.
- An openCL reflectivity calculation backend is added that can use a GPU.
- Added a `reflect.use_reflect_backend` function to choose between the backends
used to calculate reflectivity.

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- allow master and slave chopper parameters to be ignored in PLP reduction.
- tunable t_0 offset in PLP/SPZ reduction.
- possibly_create_parameter accepts default bounds/vary/constraint. This fixes
a bug in MixedSlab.
- Don't require numpy be installed before setup.py can run.
- Speed up reflectivity calculation if there is solvation.
- reflect.choose_dq_type for finding out fastest mode of resolution smearing.
- User can now select resolution smearing approach in ReflectModel. Choosing
between 'pointwise' or 'constant'.
- Add progress bar for Curvefitter.fit() (requires tqdm being installed).
- A few micro-optimisations.
- Optimized pickling/unpickling of Bounds instances. This can lead to a huge
performance increase (~ 40%) when doing parallel sampling.
- MCMC sampling initialisation made reproducible

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- Fixed a bug in the GUI that prevented load/save of mixed area models.
- Fixed a bug in the resynthesis of data.
- Fixed a bug in the loading of event data from the Platypus monobloc detector.
- Added a FresnelTransform.
- Document inequality constraints.
- Made refnx SLD calculator GUI more fault tolerant to incorrect formula.
- Add example Jupyter notebook for batch reduction.
- Autoscaling for Panalytical XRR reduction.
- Fixed bug in drag/drop within a Stack in the GUI.
- Added a MixedSlab Component which is constituted from several individual
Scatterers. The SLD of the MixedSlab is weighted by their volume fractions.

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- Prevent spurious benchmark package installation.
- Fixed bug when loading a MTFT file saved in a previous version of refnx.
- event mode data reduction sped up by an order of magnitude
- Added neutron transmission calculator (if periodictable is present)
- event file reader can now read any ANSTO packedbin file (reduction).
- Align SLD plots around a specific interface in a slab representation. Useful
if plotting many samples at the same time.
- Add MaterialSLD object that is constructed from a chemical formula and mass
density. This enables the use of specific materials to describe layers, e.g.
MaterialSLD('SiO2', density=2.2).
- Components can be multiplied by an integer to make them repeat.
- Add _open_mp_helpers to MANIFEST.in (gh381).
- update bundled vendored emcee