Fix bug for integer IDs when using rsmxval

[desilinguist]

Closes #570.

Convert the ID column to string when reading in the training data file in rsmxval so that it matches the rest of the rsmtool pipeline. Also, add a new test for this use case.

[codecov]

Codecov Report

Base: 95.75% // Head: 95.75% // No change to project coverage 👍

Coverage data is based on head (b33347a) compared to base (a77d3f2).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #577   +/-   ##
=======================================
  Coverage   95.75%   95.75%           
=======================================
  Files          29       29           
  Lines        4000     4000           
=======================================
  Hits         3830     3830           
  Misses        170      170           

Impacted Files	Coverage Δ
rsmtool/utils/cross_validation.py	95.34% <100.00%> (ø)

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