Add UCSC TxDb

[gamazeps]

Work for issue #11

This is currently a very dirty WIP, the doc is missing, the notebook is in the wrong place and will be turned into a cookbook

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Codecov Report

Attention: Patch coverage is 22.95082% with 141 lines in your changes are missing coverage. Please review.

Project coverage is 68.39%. Comparing base (a18e2fb) to head (ca0c28f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #66       +/-   ##
===========================================
- Coverage   93.09%   68.39%   -24.71%     
===========================================
  Files           6        8        +2     
  Lines         333      522      +189     
===========================================
+ Hits          310      357       +47     
- Misses         23      165      +142     

Files	Coverage Δ
src/genomic_features/__init__.py	100.00% <100.00%> (ø)
src/genomic_features/ucsc/__init__.py	100.00% <100.00%> (ø)
src/genomic_features/ucsc/ucscdb.py	21.66% <21.66%> (ø)

... and 1 file with indirect coverage changes

[gamazeps]

Still a massive WIP, will finish this Weekend (the bottleneck right now is finding which are the joins done to provide the tables in the R implementation)