Mantid: use Euphonic for force constant import to SimulatedDensityOfStates algorithm

[ajjackson]

The SimulatedDensityOfStates Algorithm in Mantid reads frequencies and optionally eigenvectors from Castep output files, providing a few useful plotting options.

It would be helpful to permit import of force constants data using Euphonic's import features:

• to provide support for a wider range of atomistic codes (via Phonopy)
• to allow dense q-point sampling of Brillouin Zone even if this was not in the original calculation data

This will require packaging of Euphonic with Mantid, which will likely introduce some challenges of its own. By targeting this relatively simple and unused Algorithm we can solve those problems while avoiding entanglement with the more complex Abins Algorithm.

As such this soft-blocks Abins/Euphonic integration tasks #67 #68

[ajjackson]

Work has started on this, but it is possibly blocked by #167

[ajjackson]

This code is now reasonably functional, but cannot be accepted until the packaging situation is developed.
mantidproject/mantid#30760

This can now be considered blocked by #167

[ajjackson]

Mantid PR is in review, looks good. mantidproject/mantid#30760