Band names : always use strings instead of enums

[remi-braun]

Right now, band names are either enums (children of BandNames, i.e. SpectralBandNames) or strings for spectral indices.
This gives inconsistency in the datasets : accessing a band has to be done through the object even if a string is given as input.
It may be easier for everyone to downgrade the BandNames to pure strings with side functions.

Related to #227