Remove DataSet.append() due to upcoming deprecation in Pandas

[JBGreisman]

This PR fixes #146 to remove the overloaded function, DataSet.append(), and any uses of it.

[codecov-commenter]

Codecov Report

Merging #149 (c040f92) into main (f7d2a2c) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #149      +/-   ##
==========================================
- Coverage   98.46%   98.45%   -0.02%     
==========================================
  Files          43       43              
  Lines        1696     1681      -15     
==========================================
- Hits         1670     1655      -15     
  Misses         26       26              

Flag	Coverage Δ
unittests	98.45% <100.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
reciprocalspaceship/concat.py	100.00% <ø> (ø)
reciprocalspaceship/dataset.py	98.00% <100.00%> (-0.08%)	⬇️
reciprocalspaceship/utils/stats.py	100.00% <100.00%> (ø)

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[kmdalton]

This looks good to me. I was not able to find any other places in the code base with offending append statements. To the best of my knowledge, this is complete. The straightforward substitution with rs.concat seems the most natural route too. Sorry for the late reply!