feat: ability use builtin observatories

[steven-murray]

Adds several default builtin observatories.

[codecov]

Codecov Report

Merging #75 (b18e66d) into main (0c6a344) will decrease coverage by 0.07%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
- Coverage   97.74%   97.66%   -0.08%     
==========================================
  Files          12       12              
  Lines         887      900      +13     
==========================================
+ Hits          867      879      +12     
- Misses         20       21       +1     

Impacted Files	Coverage Δ
py21cmsense/observatory.py	96.13% <92.85%> (-0.30%)	⬇️

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ca6a483) 98.59% compared to head (aecb156) 98.60%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
+ Coverage   98.59%   98.60%   +0.01%     
==========================================
  Files          15       15              
  Lines        1067     1076       +9     
==========================================
+ Hits         1052     1061       +9     
  Misses         15       15              

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