DataSet.write_mtz() should not move unmerged reflections to reciprocal ASU

[JBGreisman]

This PR fixes #110 to ensure that unmerged reflections are not moved to the reciprocal ASU after a call to DataSet.write_mtz().

[codecov-commenter]

Codecov Report

Merging #111 (277a199) into main (9d9b052) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #111   +/-   ##
=======================================
  Coverage   98.72%   98.72%           
=======================================
  Files          41       41           
  Lines        1569     1574    +5     
=======================================
+ Hits         1549     1554    +5     
  Misses         20       20           

Flag	Coverage Δ
unittests	98.72% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
reciprocalspaceship/io/mtz.py	100.00% <100.00%> (ø)

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[JBGreisman]

The most recent commit also extends this PR to support invoking DataSet.write_mtz() or DataSet.to_gemmi() with unmerged data that is already in the reciprocal ASU. Previously, such data would have been re-mapped back to the observed indices upon a call to write_mtz() or to_gemmi()