fix type

rangecut implements wavenumbers_ when cut

Update sklearn api (#60)

* update interface in air_pls

* update airpls

* update arpls api

* fix documentation in index_selector

* fix typo in index_selector

* fix api in constant baseline correction

* update cubic splines

* update api in linear correction

* update init for linear correction

* fix api in non negative

* fix api in polynomial and in subtract reference

* fix norris williams

* fix api in savitzky golay

* fix minmax scaler

* fix norm scaler

* fix point scaler

* fix emsc

* update msc

* update rnv

* update snv

* update mean filter

* update median filter

* update savgol filter

* update whittaker

Feature selection (#59)

* change module name to feature_selection

* add range_cut_selector

* update tests for rangecut selector

* refactor range cut selector

* Add index selector

* test index selector

* Remove old range_cut and select_features

* upate depricated np.matrix by standard np.array, and adjust code accordingly

* rename funcitons in feature selection to follow sklearn

update release yml

make title bold

test raise error in emsc

add tests to select features

update min max plot

add explanations about rangecut