Beautify HoeffdingTreeRegressor

[AlbandeCrevoisier]

Used for reference.

Minor improvements:

• use iterators instead of index
• return right at the if test instead of shoving it back to an else and indent all the code
• ...

---

Checklist

• Code complies with PEP-8 and is consistent with the framework.
• Code is properly documented.
• Tests are included for new functionality or updated accordingly.
• Travis CI build passes with no errors.
• Test Coverage is maintained (threshold is -0.2%).
• Files changed (update, add, delete) are in the PR's scope (no extra files are included).

[codecov]

Codecov Report

Merging #194 into master will decrease coverage by 0.03%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master     #194      +/-   ##
==========================================
- Coverage   85.11%   85.08%   -0.04%     
==========================================
  Files         173      173              
  Lines       12836    12838       +2     
==========================================
- Hits        10926    10923       -3     
- Misses       1910     1915       +5

Impacted Files	Coverage Δ
src/skmultiflow/trees/hoeffding_tree_regressor.py	83.7% <90.47%> (-0.3%)	⬇️
src/skmultiflow/meta/online_rus_boost.py	85.71% <0%> (-1.79%)	⬇️
src/skmultiflow/trees/isoup_tree.py	84.39% <0%> (-0.36%)	⬇️