RF Variable Importance

[Benjamin Jakimow]

As it is requested by more and more EnMAP-Box users, we should add a plot of the RF variable importance
(was already part of the IDL EnMAP-Box)

https://bitbucket.org/hu-geomatics/enmap-box-idl/wiki/imageRF - Manual for Application#!variable-importance-of-rfcrfr-models