Transfer managers for icosahedral sphere mesh

[JHopeCollins]

The block solves for the non-linear shallow water equations example needs a bespoke transfer manager for the icosahedral sphere mesh.
This manager deals with the fact that refinements of the icosahedral sphere are not nested, so the fine mesh coordinates need to be projected down onto the surface of the coarse mesh to carry out the transfers.

This is probably general enough that it could/should be supported by Firedrake directly. If this happens, we can remove the file in the examples directory that currently defines this transfer manager.

[JHopeCollins]

This functionality has now been moved to the utils library in utils.mg
See pull request #35

The plan is still to propose it to be included in Firedrake, but for now we no longer need to carry the mg file around in the examples folder.