add comments to feature_tree

[RogerTangos]

• describe what some of the instance variables are
• Add one line break in code, bringing max line size to 100
• rename all_features in __init__, since it's confusing to have all_features be different from self.all_features

[codecov]

Codecov Report

Merging #400 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #400   +/-   ##
=======================================
  Coverage   95.98%   95.98%           
=======================================
  Files          91       91           
  Lines        7916     7916           
=======================================
  Hits         7598     7598           
  Misses        318      318

Impacted Files	Coverage Δ
...eaturetools/computational_backends/feature_tree.py	100% <100%> (ø)	⬆️

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[kmax12]

Looks good! I suspect more work can be done here, but need to start somewhere. Thank you for the contribution. Merging now