Deprecate (and eventually remove) << and >> IO syntax

[Prof Garth Wells]

The << and >> syntax for IO does not follow common convention and is not suited to non-trivial output cases, e.g. cases with multiple arguments.

Notes:

• XDMFFile does not support the syntax (XDMF is being moved towards as the default format for DOLFIN).

• Early task is to provide read/write functions for VTK formats.

• Move demos to using read/write.

Removal will be long-term as this is a very long-standing syntax.