Proper handling of @import directive inside a file read using pkg_resources

load_definitions() takes a new parameter to indicate if the file was loaded
from as a pkg_resource. In that case, related files will be opened in the
same way.

See #68 and ufo-kit/concert#94
Fix get_dimensionality for non multiplicative units and provides test cases

close #66
Merge branch 'master' into develop
Fix get_dimensionality
Merge branch 'master' into develop