Convert pandas/apache parquet to vaex-compatible format

[olgabot]

Read in a pandas/apache parquet file, output a vaex-compatible hdf5, which opens lazily, halving the read and write time

[codecov-io]

Codecov Report

Merging #25 into master will increase coverage by 2.11%.
The diff coverage is 48.78%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   45.83%   47.95%   +2.11%     
==========================================
  Files          20       21       +1     
  Lines        1213     1345     +132     
==========================================
+ Hits          556      645      +89     
- Misses        657      700      +43

Impacted Files	Coverage Δ
khtools/commandline.py	86.66% <100%> (+2.05%)	⬆️
khtools/pandas2vaex.py	46.15% <46.15%> (ø)
khtools/compare_kmer_content.py	60.73% <0%> (-0.02%)	⬇️
khtools/sequence_encodings.py	98.82% <0%> (+0.82%)	⬆️

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[olgabot]

Not doing this now - low priority