A test of fracture flow added.  Fixes #3421

This test is a benchmark against an analytic
solution for solute advection through a fracture
and transverse diffusion through the bulk.

Both a heavy and a "light" test have been added that
are identical except for the timestep used.

Comprehensive doco has been added.
Merge pull request #3428 from WilkAndy/fracture_flow_3421

A test of fracture flow added.  Fixes #3421
Merge commit '085268c51442c242ed13dd584d8705ee80276dab'