I think this should use
```
S = shear._shear.getMatrix()
```
I think it might be a stabler calculation, particularly at small shear values.  It also may already be calculated, since the shear object caches this matrix.  So it will be more efficient.  

Note: the wrapping of this function had been commented out, since we didn't have a use for it in python until now.  I just uncommented it, so it is available again.