added: spec file for redhat packaging

note that to actually build the RPM you need
1) the epel repository enabled
2) several additional libraries i had to package (alberta, alugrid,
dune-xxx). these will be made available in the OPM yum repository

i chose to stick to the redhat standards, where you have a meta package (ewoms). usually this depends on the library, but since this is templates only i have made the meta package depend on the ewoms-devel instead.