Explore using filters for WCS TIME requests

[RousseauLambertLP]

I think we could update our code to use the FILTER key in MapServer fot the WCS time requests instead of using band names.

This could also mean that we would not need to have a mapfile for WMS and another one for WCS.

See https://www.mapserver.org/uk/ogc/wcs_server.html#spatio-temporal-indexes

So basically we could add a filter in the tileindex layer and use mapscript for replacing the filter value with the parameter passed in the request.

We would have to test how many filters we can use because at the moment, using the band names allows us to ask for up to 256 bands (time step) at the same time.

@tomkralidis any thoughts or comments ?