RF: Parse bval/bvec files uniformly, add n_rows attribute to associations

[effigies]

This PR is paired with bids-standard/bids-specification#1622, which proposes augmenting the bval/bvec associations in the context.

This simplifies things a bit by using the same code to parse bvals and bvecs, ensuring that bugs found and fixed in one will work for the other.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (6789e4d) 85.58% compared to head (1f1cf20) 83.59%.

❗ Current head 1f1cf20 differs from pull request most recent head 0f273ff. Consider uploading reports for the commit 0f273ff to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1813      +/-   ##
==========================================
- Coverage   85.58%   83.59%   -1.99%     
==========================================
  Files         132       92      -40     
  Lines        6728     3895    -2833     
  Branches     1548     1272     -276     
==========================================
- Hits         5758     3256    -2502     
+ Misses        862      541     -321     
+ Partials      108       98      -10     

see 40 files with indirect coverage changes

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