Add location of subarray reference point to SubarrayDescription

[maxnoe]

No description provided.

[kosack]

yes, this has been missing for a while!

It will make the alt/az → ICRS transformations nicer, and also maybe later we can add a map imagery overlay to the ArrayDisplay now that we also have the EastingNorthingFrame. I hacked a version at one point:

[codecov]

Codecov Report

Merging #1980 (f3165fb) into master (5341875) will increase coverage by 0.02%.
The diff coverage is 95.20%.

@@            Coverage Diff             @@
##           master    #1980      +/-   ##
==========================================
+ Coverage   92.18%   92.20%   +0.02%     
==========================================
  Files         193      193              
  Lines       15238    15447     +209     
==========================================
+ Hits        14047    14243     +196     
- Misses       1191     1204      +13     

Impacted Files	Coverage Δ
ctapipe/instrument/subarray.py	91.66% <73.91%> (-1.93%)	⬇️
ctapipe/io/tableloader.py	96.52% <98.83%> (+1.04%)	⬆️
ctapipe/conftest.py	94.25% <100.00%> (-0.23%)	⬇️
ctapipe/instrument/tests/test_subarray.py	100.00% <100.00%> (ø)
ctapipe/io/tests/test_hdf5eventsource.py	97.34% <100.00%> (ø)
ctapipe/io/tests/test_simteleventsource.py	100.00% <100.00%> (ø)
ctapipe/io/tests/test_table_loader.py	99.42% <100.00%> (+0.36%)	⬆️
ctapipe/utils/index_finder.py	85.71% <0.00%> (-3.58%)	⬇️
ctapipe/io/astropy_helpers.py	90.98% <0.00%> (-1.64%)	⬇️
ctapipe/image/hillas.py	98.50% <0.00%> (-1.50%)	⬇️
... and 6 more

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