Add API reference docs

[leferrad]

This PR solves #7 by adding API docs generated with Sphinx, and deployed with ReadTheDocs. Also solving some aspects in the code:

• Improved docstrings
• Removing unnecessary method _get_predictions in classes
• Solved Use a sparse matrix of np.eye in fit_woodbury #4 by using sparse eye() method in fit_woodbury
• Solved Add partial_fit() methods as alias for fit() in pyoselm.oselm classes  #16 by adding partial_fit() methods as alias for fit() in pyoselm.oselm classes

[codecov]

Codecov Report

Merging #17 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #17   +/-   ##
=======================================
  Coverage   99.75%   99.75%           
=======================================
  Files           4        4           
  Lines         408      408           
=======================================
  Hits          407      407           
  Misses          1        1           

Impacted Files	Coverage Δ
pyoselm/layer.py	100.00% <ø> (ø)
pyoselm/elm.py	100.00% <100.00%> (ø)
pyoselm/oselm.py	99.00% <100.00%> (+0.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d033d8f...197e33a. Read the comment docs.