Add observatory location to SubarrayDescription

[kosack]

the SubarrayDescription (built currently in each EventSource) should contain the location of the observatory as an astropy.coordintates.EarthLocation. This is needed e.g. for some coordinate transforms.

I suggest we add it as a field called "location", so we can do things like:

subarray = SubarrayDescription(...)
elevation = subarray.location.height
# <Quantity 2124. m>

[kosack]

see #1030

[maxnoe]

With this added, we should also enable transformations to/from GroundFrame to ITRS:
https://docs.astropy.org/en/stable/api/astropy.coordinates.ITRS.html

Which makes it easy to get individual earth locations for each telescope

[maxnoe]

Done in #1980