[IMPORTED] Add a convention dict to the orientations parameter

[aplowman]

Imported from: LightForm-group/UoM-CSF-matflow#13

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We are now tracking some important conventions regarding orientation data (as part of the orientations parameter dict), including the unit_cell_alignment and the P constant. However, these two conventions are still missing:

• active vs passive
• crystal-to-specimen or specimen-to-crystal

To that end, it would be sensible to dump all conventions into a convention dict in the orientation parameter.