Refactoring, small improvements, and cleaning notation of IsotonicRegression
Changing docs from "vector" to "1d array"
Reverting array X to vector x rename
Docstring addition and x->X
Incorporating @ogrisel's comments for simplification
More simplifications
Fixing simplification for clip
Adding test coverage for weight deprecation in IsotonicRegression and isotonic_regression
Improving exception and warning coverage
Merge pull request #3250 from mjbommar/isotonic-refactor-2

[MRG+1] Refactor and clean IsotonicRegression
Included perlimenary code for basicly functional sparse support on top of the rebase