Handle multiband images properly

[ytanguy]

SLURP takes 4 bands images as input, but quite often it could be interesting to use more bands (ex : PNEO images).

It needs several modification :

• prepare.py : compute validity mask as the intersection between each band valid mask
• *_mask.py : allow algorithms to use more than 4 bands (ie : improve eomultiprocessing engine :)

[namsellem]

@ytanguy Handling of validity masks is proposed through PR #67, Still some debug to perform

Further devs will be necessary in order to have *_mask.py using more than 1 bands (only prepare uses im_vhr --> 4 bands)

[namsellem]

Test implementation of n bands through PR in watermask.py Random Forest implementation #76