Boresight and field of view intersection on the surface

• expose spiceypy.getfov attributes to SPICE reference: 37717a45, 69209627, 2ff62536, 47573579
• In the general case, the insturment boresight intersection on the surface is distinct from the sub-spacecraft point: afd3f373, 70770c60, afd3f373, 4ae2bb3a, 90e92181
• the boresight surface intersection can be performed spiceypy.sincpt (throw spiceypy.stypes.NotFoundError on the limb): 2fefbdb5, 70770c60, 2ff62536, cd333b5b
• the foorprint of the field of view contour needs to be expended based on its shape (CIRCLE, RECTANGLE…) and the points not intersecting the surface need to be projected on the limb edge: 37717a45, 1f3adff7, cd333b5b
• to improve the performance (and to avoid to draw unnecessary polygons), it will be more relevant to display only the 2 points with the max extent orthogonal to the speed vector: 1f3adff7, 422a8405