Provide DTD schema to describe parameter file format

After the discussion about the use of JSON versus XML, it struck me that I haven't seen any format definition of the parameter file format, so here is one that I whipped up quickly. It passes tests/testdata.xml and tests/extratestdata.xml:

    xmllint --dtdvalid opm/core/utility/parameters/param.dtd --noout tests/testdata.xml

If anyone has more parameter files to run through, that would be nice.