Add support for Union{Nothing, T}

[ziotom78]

This small PR adds basic support for maps based on the type Union{Nothing, T} to signal missing pixels. It's a more natural and effective way than using UNSEEN, and it requires very few adjustments to the code:

• A special constructor for HealpixMap
• Basic support for map I/O (currently only floating-point values are supported, because I'm lazy!)
• Documentation
• Tests

[codecov]

Codecov Report

Merging #63 (e81932c) into master (5d60214) will increase coverage by 0.20%.
The diff coverage is 92.00%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
+ Coverage   86.30%   86.50%   +0.20%     
==========================================
  Files          18       18              
  Lines         942      956      +14     
==========================================
+ Hits          813      827      +14     
  Misses        129      129              

Impacted Files	Coverage Δ
src/map_io.jl	90.00% <91.30%> (+2.06%)	⬆️
src/map.jl	63.33% <100.00%> (+2.61%)	⬆️

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