Add image index to each row when reading images

[JulienPeloton]

Thanks to #91, I realized that the user completely loses the data order once the data read from spark-fits is written on disk. While this has no impact for tables (rows are independent), this matters a lot for images!

This PR introduces image index to easily track the provenance.

Note: if this is insufficient, we can add another column with row position within an image. But let's wait if this is really needed.

[codecov-commenter]

Codecov Report

Merging #97 into master will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
+ Coverage   90.33%   90.49%   +0.15%     
==========================================
  Files           9        9              
  Lines         559      568       +9     
  Branches       70       74       +4     
==========================================
+ Hits          505      514       +9     
  Misses         54       54              

Impacted Files	Coverage Δ
.../com/astrolabsoftware/sparkfits/FitsHduImage.scala	78.78% <100.00%> (+1.36%)	⬆️
.../astrolabsoftware/sparkfits/FitsRecordReader.scala	77.68% <100.00%> (+1.37%)	⬆️

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