Fix AttributeError when calling isUnlimited()

This has been fixed for some time in cdat_lite.  An example test which exposes
the bug would be as follows (assuming ncfile exists with variable 'climseas'):
```
def test_dimension_isUnlimeted():
    f = cdms2.open(ncfile)
    try:
        v = f.variables['climseas']
        t = v.getTime()

        # fails with AttributeError: CdmsFile.dimensions
        assert t.isUnlimited()
    finally:
        f.close()
```