syntax change on trunk

I've just committed a change to trunk that may affect some of you.  
Although there are many internal changes involved, they should  
hopefully not be of interest to FiPy users. On the other hand, you can  
see the few examples that changed at

   http://www.matforge.org/fipy/changeset?new=trunk%2Fexamples%402565&old=trunk%2Fexamples%402541

Basically, the change affects how you specify faces or cells for  
setting boundary conditions and initial conditions.

The result of calls like `mesh.getExteriorFaces()` or  
`mesh.getFacesLeft()` is no longer a special `FaceIterator` class, but  
is now just a boolean mask. This makes it easy to specify a particular  
subset of faces with `&` and `|`, which could be a bit tricky in the  
old scheme. I've also done away with the `filter` argument of  
`getFaces()` and `getCells()`, as I don't think it's at all necessary  
anymore and it was massively inefficient.

The new syntax is, IMNSHO, easier to read and more powerful than the  
old, but it's not 100% compatible, so I wanted to give people a heads- 
up.

If you can't make sense of the new scheme, by all means let us know.