Tests that allow easy exploration of user objects have been added.

Fixes #3394

These tests are entirely trivial from a test-suite point of view.
They pass if they run.  They fail if the input files cause a crash.

Also a little doco has been added to user.tex that explains the
functionality.
Fixes #3411 .  Corrected uo4.i

Put some error-checking stuff in RichardsMaterial so this won't happen again
and also some LIBMESH_DIM instead of "3".

Also slightly altered RichardsSeffPrimPrimeAux.C to avoid a comparison
between signed and unsigned, which was not the cause of this problem, btw.
Merge pull request #3419 from WilkAndy/uo4_crash_3411

Fixes #3411 .  Corrected uo4.i, also slightly altered RichardsSeffPrimPr...
Merge pull request #3402 from WilkAndy/uo_eg_3394

Tests that allow easy exploration of user objects have been added.
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.