refnx/refnx: v0.1.15

0.1.15 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 sdis