Spatiallyvaryingviscosity

Changes to add parsed viscosity capability have passed tests and a new test: test_stokes_poiseuille_flow_parsed_viscosity has been added. This will further take shape as a variable viscosity test case with inputs from @roystgnr and @nicholasmalaya 
This doesn't work for out-of-source builds.

test_stokes_poiseuille_flow.sh.in doesn't go through autoconf parsing because we wanted it to look more complicated, it does so because we wanted it to work correctly.  ;-)