Fix cfunits import error on Windows

[mcflugen]

This fixes an issue on Windows where the units database could not be found (udunits2.xml). This resulted as an assertion error when importing cfunits from within pymt,

>>> import cfunits
Traceback (most recent call last):
...
    assert(0 == _ut_unmap_symbol_to_unit(_ut_system, _c_char_p(b'Sv'), _UT_ASCII))

The issue is that udunits2 library can not correctly find udunits2.xml in the default location. On Windows anyway. To fix this, I set the UDUNITS_XML_PATH environment variable in pymt/__init__.py to point to the path of udunits2.xml within the cfunits package. I do this for all platforms, not just Windows but I think that should be fine.

[coveralls]

Coverage increased (+0.03%) to 71.379% when pulling b583c44 on mcflugen/fix-cfunits-import-error-on-windows into cd191be on master.