test all methods of Min.pm and rename constructor

Change the old constructor to new_from_xml so that there can be a
separate new constructor which doesn't require XML input. Test
all accessors of the TBX::Min class. Rename 04-read_xml.t to come
after the tests for new and all of the accessors.
moved subject_field from document into concept

Because the subject_field is normally contained at the concept level in
other dialects of TBX, it has been moved into the concept of TBX-Min.
This required updating the sample file, and then updating Min.pm and
ConceptEntry.pm to reflect the change in structure, and then updating
their tests.