import_scad() imports illegal python identifiers as module names from filenames

```
>>> mcad = solid.import_scad("MCAD")
>>> dir(mcad)
['2Dshapes', '3d_triangle', '__class__', '__delattr__', [...] 'units', 'unregular_shapes', 'utilities']
>>> mcad.2Dshapes
  File "<stdin>", line 1
    mcad.2Dshapes
         ^
SyntaxError: invalid syntax
```