Fix lonpole in zenithal projections

[mhasself]

geometry2() would always set lonpole=0 for zenithal projs, where geometry() would almost always set lonpole=180. This PR:

• updates geometry() so it sets lonpole=180, even if CRVAL is the north pole.
• updates geometry2() to behave the same way as geometry().
• Adds ARC and SIN projections to geometry() (these were already supported by geometry2).
• Adds test to confirm that zenithal maps are oriented as we would like, by default, with the RA=0 line intersecting the bottom of the map.

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 40.78%. Comparing base (25e607f) to head (18c1d2c).

Files with missing lines	Patch %	Lines
pixell/wcsutils.py	93.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #292      +/-   ##
==========================================
+ Coverage   39.80%   40.78%   +0.98%     
==========================================
  Files          30       30              
  Lines       10439    10457      +18     
==========================================
+ Hits         4155     4265     +110     
+ Misses       6284     6192      -92     

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