Refactored scaling functions

Added RobustScaler and MaxAbsScaler
Added *scale functions
removed duplicated code
Squash commits and rebase
Simplified MinMaxScaler
Improved tests
Removed leftover 'print' statement
Fixed comment
Documented 2nd test inside the zero-variance tests
Add note to minmax_scale
Improved wording in documentation
Fix backticks
Fixed typo
WIP: clean up test cases