Add functions to read/write a_ℓm and C_ℓ from/to FITS files

[LeeoBianchi]

Added writeAlmToFITS, writeClToFITS, readClfromFITS, brief markdown documentation chapter for the Cl-related functions.

[codecov[bot]]

Codecov Report

Merging #91 (5878a8d) into master (b9e8d91) will decrease coverage by 6.26%. The diff coverage is 0.00%.

:exclamation: Current head 5878a8d differs from pull request most recent head 097dec7. Consider uploading reports for the commit 097dec7 to get more accurate results

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
- Coverage   87.43%   81.17%   -6.27%     
==========================================
  Files          19       20       +1     
  Lines        1154     1243      +89     
==========================================
  Hits         1009     1009              
- Misses        145      234      +89     

Impacted Files	Coverage Δ
src/Cl.jl	0.00% <0.00%> (ø)
src/Healpix.jl	100.00% <ø> (ø)
src/alm.jl	56.03% <0.00%> (-40.99%)	:arrow_down:

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[LeeoBianchi]

From the test log it looked like a bracket was missing in alm.jl, it should work now.