add Gnomonic (tangent plane) projection

[xzackli]

This PR adds basic support for the Gnomonic (tangent plane) projection.

[codecov-commenter]

Codecov Report

Base: 89.94% // Head: 88.67% // Decreases project coverage by -1.26% ⚠️

Coverage data is based on head (6697acd) compared to base (659d96b).
Patch coverage: 84.52% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
- Coverage   89.94%   88.67%   -1.27%     
==========================================
  Files           8       11       +3     
  Lines         905      954      +49     
==========================================
+ Hits          814      846      +32     
- Misses         91      108      +17     

Impacted Files	Coverage Δ
src/Pixell.jl	100.00% <ø> (ø)
src/enmap.jl	69.76% <0.00%> (-3.95%)	⬇️
src/enmap_ops.jl	81.25% <33.33%> (-10.54%)	⬇️
src/projections/tan_proj.jl	75.60% <75.60%> (ø)
src/projections/arbitrary_wcs.jl	90.10% <90.10%> (ø)
src/projections/car_proj.jl	90.67% <90.67%> (ø)

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