Warning if no WCS is specified

[msyriac]

This fixes #274

[codecov[bot]]

Codecov Report

:x: Patch coverage is 0% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 37.32%. Comparing base (34d03ba) to head (9b2d442).

Files with missing lines	Patch %	Lines
pixell/enmap.py	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #317      +/-   ##
==========================================
- Coverage   37.33%   37.32%   -0.01%     
==========================================
  Files          37       37              
  Lines       11465    11466       +1     
==========================================
  Hits         4280     4280              
- Misses       7185     7186       +1     

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