Use solar_angle_equivalency and Helioprojective.is_visible()

[wtbarnes]

Fixes #94
Fixes #91

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 61.53846% with 5 lines in your changes missing coverage. Please review.

Project coverage is 51.24%. Comparing base (fde01b8) to head (ec1d3bc).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
synthesizAR/instruments/base.py	42.85%	4 Missing ⚠️
synthesizAR/visualize/fieldlines.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
+ Coverage   51.14%   51.24%   +0.09%     
==========================================
  Files          40       40              
  Lines        2100     2092       -8     
==========================================
- Hits         1074     1072       -2     
+ Misses       1026     1020       -6     

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