🐛 need change 'bands' into 'indexes' in DEM class

[liuh886]

Bug

The DEM class in xdem/dem.py has following lines:

[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")

Error

it raises
AttributeError: 'DEM' object has no attribute 'nbands'

Reason

The 'bands' were changed into 'indexes' since geoutils 0.0.10.