Add HDU and header copy functions

[jishnub]

Add the functions fits_copy_file that can copy multiple HDUs from one file to another, fits_copy_hdu that copies only the current HDU, fits_copy_header that copies only the header from the current HDU and appends to the output file, and fits_copy_data that copies the data but not the header.

[codecov]

Codecov Report

Merging #17 (a444459) into master (5c376e4) will increase coverage by 0.71%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   77.04%   77.75%   +0.71%     
==========================================
  Files           1        1              
  Lines         588      616      +28     
==========================================
+ Hits          453      479      +26     
- Misses        135      137       +2     

Impacted Files	Coverage Δ
src/CFITSIO.jl	77.75% <100.00%> (+0.71%)	⬆️

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