Add deleteat

[jishnub]

This PR depends on https://github.com/JuliaAstro/CFITSIO.jl/pull/6, where the function to delete an HDU is added. Now it's possible to remove an HDU from a FITS file.

julia> f = FITS(tempname() * ".fits", "w");

julia> write(f, ones(1,1))

julia> write(f, ones(2,2))

julia> write(f, ones(3,3))

julia> deleteat!(f, 2)
File: /tmp/jl_XeX9WX.fits
Mode: "w" (read-write)
HDUs: Num  Name  Type   
      1          Image  
      2          Image  

julia> size(f[2])
(3, 3)

julia> deleteat!(f, 1) # deleting the primary HDU recreates one with minimal metadata and no image
File: /tmp/jl_XeX9WX.fits
Mode: "w" (read-write)
HDUs: Num  Name  Type   
      1          Image  
      2          Image  

julia> size(f[1])
()

julia> size(f[2])
(3, 3)

[codecov[bot]]

Codecov Report

Merging #156 (e0aaa91) into master (95780ef) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
+ Coverage   91.35%   91.42%   +0.06%     
==========================================
  Files           5        5              
  Lines         613      618       +5     
==========================================
+ Hits          560      565       +5     
  Misses         53       53              

Impacted Files	Coverage Δ
src/FITSIO.jl	100.00% <ø> (ø)
src/header.jl	85.88% <ø> (ø)
src/fits.jl	95.83% <100.00%> (+0.31%)	:arrow_up:

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[jishnub]

@giordano @mileslucas Could you have a look at this PR?