Migrate highness plugin functionality to core NCrystal

[tkittel]

Highness plugins developed by @ShuqiXu7, @nicriz, @marquezj, @dddijulio, et. al. should be reviewed for adoption into core NCrystal.

For the SANSND plugin, perhaps we could simply have a dedicated .ncmat file with the I(Q). Once NCrystal supports I(Q) for SANS of course.

For the magscat plugin, we simply need to add an optional section with the new parameters.

For the MD texture plugins we need to discuss, since it is useful for some applications, but does not implement a consistent cross section for off-axis neutrons. Perhaps integrate it but require a builtin '@CUSTOM_` section to activate?

Anyway, this is not super urgent, but would be a shame to drop these features. And it is hard to maintain them as separate plugins.