some improvements / fixes to read_map and also add write_map

[guanyilun]

• add exports
• move show(::WCSTransform) to upstream (WCS.jl)
• add sel option in read_map
• fix default POLCCONV to COSMO
• add write_map

[codecov-commenter]

Codecov Report

Merging #10 (776e8b0) into main (0a88668) will increase coverage by 6.94%.
The diff coverage is 11.11%.

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
+ Coverage   48.69%   55.64%   +6.94%     
==========================================
  Files           2        2              
  Lines         115      124       +9     
==========================================
+ Hits           56       69      +13     
+ Misses         59       55       -4     

Impacted Files	Coverage Δ
src/enmap.jl	51.32% <11.11%> (+8.05%)	⬆️

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