Fix for ska delete

[johnlees]

Silly mistake in the loop that does the deletion which I'm surprised wasn't caught by the tests, maybe need to review these.

Closes #57

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (4eb1446) 92.42% compared to head (247b24d) 92.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   92.42%   92.50%   +0.08%     
==========================================
  Files          16       16              
  Lines        2152     2176      +24     
==========================================
+ Hits         1989     2013      +24     
  Misses        163      163              

Files	Coverage Δ
src/generic_modes.rs	100.00% <ø> (ø)
src/merge_ska_array.rs	99.65% <100.00%> (+0.77%)	⬆️
src/ska_ref.rs	96.22% <100.00%> (-0.02%)	⬇️
src/lib.rs	80.63% <85.71%> (-0.31%)	⬇️

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