jcasgen.sh has trouble with import by name

If processing a typesystem file that uses <import name="some/path/to/blah">, and the path to blah is not relative to where jcasgen is run, jcasgen will fail because uima.datapath cannot be set.

The simplest approach to resolve it would be to read the value from an evironment variable such as UIMA_DATAPATH.