Refnx

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- build against numpy=2
- update gh runners

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- Store the fname as a PurePath in Data1D. This is because pickled datafiles
from one OS may not be unpickleable on another OS. e.g. Posix can't unpickle
WindowsPath and vice versa.
- modified `util.general.neutron_transmission` to be able to select which cross
sections are used for transmission calculation.
- We revised the ReflectReduce.write_offspecular to output an ascii file with
four columns, qz,qx,m_ref, m_ref_err.
- ReflectModelTL added. Carries out reflectivity for wavelength dependent
scattering length density profiles.

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- scale log-prior in log-posterior calculation using a multiplier. This allows
one to balance relative size of likelihood and prior.
- added Structure.from_slabs classmethod.
- fixed bug when trying to print a Parameter.

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- bump minimum Python version to 3.9.
- remove oldest-supported-numpy from list of build requirements.
- add "numba_parratt" reflectometry kernel. If numba is installed this kernel
has greater calculation speed (in calculating the reflectivity) under some
circumstances.
- fixed bug when exporting parameters from refnx GUI.
- use optionally installed black to format code fragment exported from refnx
GUI.
- update jax reflectivity kernel to fix deprecated functions.

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- remove roadblocks for building wheels for cp312.
- update vendored emcee.
- change str.format to f-strings.