For coronagraph Lyot masks, add reference information on alignment per each mask

[mperrin]

[relocated from https://github.com/spacetelescope/webbpsf/pull/675]

This PR implements a capability to have default mask alignment options (pupil_shift_x, pupil_shift_y, pupil_rotation) defined per each coronagraph mask. This is intended to allow WebbPSF to include measured in-flight information on the various masks.

Implementation details:

• in constants.py add a dict which gives values of these options per each mask, or else None to leave undefined
• in webbpsf_core.py, update code to look up and use those values.

WORK IN PROGRESS, NOT YET COMPLETE, NOT READY TO MERGE

[mperrin]

Make sure to also see the comment thread and discussion at spacetelescope/webbpsf#675