Boundary Conditions for all mesh types

[dccowan]

It seems like we can only invoke general boundary conditions on tensor meshes. This should be done for all mesh types. We should be able to impose Dirichlet, Neumann and mixed conditions on operators for Tree and Cyl meshes as well.

[lheagy]

Thanks for raising this @dccowan! There is some related discussion from way back here: #20

[jcapriot]

Closing this for the same reasons as #20