Edit bands into indexes of DEM class

[liuh886]

Edit a few lines of the DEM class in xdem/dem.py, from

[86] # self.nbands can be None when data is not loaded through the Raster class
[87] if self.nbands is not None and self.nbands > 1:
[88]     raise ValueError("DEM rasters should be composed of one band only")

to

[86] # self.indexes can be None when data is not loaded through the Raster class
[87] if self.indexes is not None and len(self.indexes) > 1:
[88]     raise ValueError("DEM rasters should be composed of one band only")

so that it fixed #339

EDIT from @rhugonnet:
Resolves #338 as well

[rhugonnet]

Thanks! Added a new test to ensure this doesn't happen again.
And for the record: actually good to test on indexes and not count (which was the equivalent of nbands), in case a single band is implicitly loaded by GeoUtils on opening (new big PR coming soon).

[adehecq]

Thanks for the edits !