allow multiple srs aliases for grids, in case a projection is named differently.

the main srs would be the one passed down to the source for rendering, whie the others would only be used to select a grid when a client requests it.

this would also allow the use of non-standard (i.e. non EPSG namespaced) projection identifiers to be used by clients, while being transformed to a standard srs id when passed to the source (i.e. the über-cool idea of using IGNF:LAMB93 for client requests could be transformed to EPSG:2154 when passed to the (wms) source).

This issue was closed by revision r233.