Improved test for OpenSCADdxf.py

As noted privately, the conditional test in "AholdsB" should be :
    ```if ((math.isclose(A.common(B).Area,B.Area,rel_tol=1e-09) == True) and (A != B)):```
rather than
    ```if ((A.common(B).Area == B.Area) and (A != B)):```

EDIT:  That also requires an ```import math``` at the top of the file

mconsidine