First draft of unsupported fillValue handing

[ndp-opendap]

This PR adds the function:

void check_fillValue_attribute_for_unsupported_types(xml_attribute attr);

To the DMZ.cc and it is called by DMZ::process_chunks().

This new function throws an exception when the value of the dmrpp/@fillValue attribute is any of:

• "unsupported-string"
• "unsupported-array"
• "unsupported-compound"
• "unsupported-variable-length-string"

This causes 7 of the dmrpp_module/tests: 139, 140, 193, 194, 195, 196, 197 to fail. In every case the new function found "unsupported-string"

I'm going to look at eliding the offending variable from the DMR.