Provide an interface to abstract different methods in OpSimSummary to get ObsHistIDs for a tileID

[rbiswas4]

Currently there are two ways envisaged for obtaining the ObsHistID values for a tile ID.

1. using a computation from a dataframe representing the opsim output which is implemented in opsimsummary.HealpixelizedOpSim.obsHistIDForTile() and requires the opsim output as a dataframe as input.
2. Precomputed results of 1. stored in a database.

The idea is that there should be a common way of reading both of these and providing obsHistID for a tileID for both of these methods. Create a class MapTile2Pointing in healpix.py to do this.

[rbiswas4]

This will go to SNsims directly.

[rbiswas4]

Implemented in SNsims rather than here, and is closed through rbiswas4/SNsims#10