Definition of detector coordinate systems

[maxnoe]

There has already been some discussion here #60 and probably else where.

ctapipe now has definitions for the TelescopeFrame and NominalFrame which are astropy sky offset frames of the Horizon (AltAz) system with respect to the individual telescope pointing (TelescopeFrame) or some arbitrary origin, i.e. the array mean pointing (NominalFrame).

https://docs.astropy.org/en/stable/api/astropy.coordinates.SkyOffsetFrame.html

I don't know if these are in their final form yet.

@kosack there was a document about coordinate frame definitions in CTA, has there been any progress on that?