Make functions for removing and updating a band or several bands

[mortenwh]

This can be useful for instance in openwind if we want to change the wind direction and recalculate the wind speed.

[asumak]

vrt.delete_band() or vrt.delete_bands() remove band(s). Is it ok to delete this issue?

[mortenwh]

I proposed a function that replaces band data with updated data, e.g. called "update_band" or something.

def update_band(array):
    meta = self.get_metadata(...)
    vrt.delete_band
    vrt.add_band (with metadata)

[asumak]

Fixed in 580d0a6.