Add xmllint-ing of SDFs to offline_check_fortran_vs_metadata.py

[peverwhee]

Description

It would be nice if the offline fortran vs metadata checker also linted the XML SDFs to validate those.

Solution

We should be able to use this function (in scripts/parse_tools/xml_tools.py) to validate any SDFs on the path:

def validate_xml_file(filename, schema_root, version, logger,
                      schema_path=None, error_on_noxmllint=False):