var_and_units_pattern (re.Pattern) as a modifiable attribute

[FObersteiner]

Pull Request Checklist:

• [x] This PR addresses an already opened issue (for bug fixes / features)

  • This PR fixes issue #55

• [x] Tests for the changes have been added (for bug fixes / features)

• [ ] Documentation has been added / updated (for bug fixes / features)

• [ ] HISTORY.rst has been updated (with summary of main changes)

• [ ] I have added my relevant user information to AUTHORS.md

• What kind of change does this PR introduce?: new feature: default regex used to parse vnames can be changed after the na file object is created.

• Does this PR introduce a breaking change?: no API change, no new dependencies.

• Other information: see also issue #45 for discussion.

[agstephens]

Thanks @FObersteiner