Recognize mandatory and optional WPS parameters to make module config more flexible

[christianbraun]

iGUESS should be aware of mandatory (min_occurs=1) and optional (min_occurs=0, max_occurs>=1) WPS input parameters.
Currently, it is not possible to run a module config if not all parameters are set, even tough they are marked as optional (see min-/max_occurs) by the WPS GetCapabilites.
E.g., it would be nice to have the possibility to put an optional mask map for an interpolation module or taking default values into account if the user doesn't specify anything in the module config.