Feature chebfun2 pde

Closes #699. 

This branch implements the `chebop2` class for solving partial differential equations on rectangular domains. This should be seen as an experimental feature. Lots more work is required to get `chebop2` to be as powerful and general as we would like.  Still, I think this is an important step in the right direction and by putting it in the development branch I'm sure it will be developed further with perhaps @Hadrien-Montanelli being the key guy.

A warning is displayed when the first chebop2 is constructed telling the user that this is an experimental feature.

Thanks to @asgeirbirkis for writing the first implementation of `adchebfun2`.