Enable SQL View parameters to be specified per layer

Enhance WMS so that Parametric SQL Views can accept parameters for individual layers (instead of applying the same set to all layers). The current VIEWPARAMS syntax would be extended to allow for specification of multiple parameter sets in the form of VIEWPARAMS=PARAMS1[,PARAMS2[...]] where the number of parameter sets provided can be zero, one or N where N is equal to the number of LAYERS specified. If only one set of parameters is specified it is applied to all layers. I have attached a patch that implements this behavior (n.b. the patch includes some overlap [KvpUtils] with the one provided in GEOS-4180).