var_and_units_pattern (re.Pattern) as a modifiable attribute

[FObersteiner]

this is a follow-up to #45

I'd like to implement var_and_units_pattern as an attribute with a @setter so I can modify it after creating the na file object with openNAFile.

This involves changes to na_core.py - I'd like to accompany the modification with some minor refactoring.

[agstephens]

@FObersteiner I have merged PR #56. Can we close this now?

[FObersteiner]

yes, thanks a lot.