Add black spot geometry interface

The geometry of the black spots will be added to some pfs_instdata data file. Add a functional interface in pfs_utils.

This geometry should be in mm in the PFI focal plane. For each spot we need to know:

• ID (presumably the ID for the cobra that can reach that spot)
• centre (x, y)
• radius r1 where the spot starts to shadow the cobra
• radius r2 where the cobra is fully shadowed

Initially, we can set r1 and r2 to be the same.

Additionally, the centre (x, y) is thought to be a function of telescope elevation (we think that it does not depend on the PFI rotation angle). So we need an API which returns a data structure (say SpotGeom) capturing these parameters, and an API to return either a
single SpotGeom or a list or dict of SpotGeom for all the spots. If the ID is a simple integer then the list can be indexed by that integer (with a None for any missing IDs, which is probably only 0). If we're using a more complicated name, then return a dict.