[material][platonicbody] use single ingroup

the doxygen build in the CI raised warnings from this header, because apparently aliases cannot be defined in multiple groups? The log says, for instance

warning: Member TwoPLocalRulesPlatonicBodyDefault found in multiple @ingroup groups! The member will be put in group PoreNetworkModels, and not in group Fluidmatrixinteractions

So apparently doxygen takes the last appearing group. I changed this to use the fluidmatrixinteractions group, as in principle, one could use these classes independent of a pore network model (for whatever reason)?

This is what the doxygen docs say about grouping:

"Note that compound entities (like classes, files and namespaces) can be put into multiple groups, but members (like variable, functions, typedefs and enums) can only be a member of one group (this restriction is to avoid ambiguous linking targets)."