Fix failing static extension example for coupled elasticity and fluid pressure

Switching to the direct linear solver allows the problem to solve.

Also switched to using the equations Jacobian calculation instead of
PETSc finite differencing, which improved the solve time from 1 min 55 seconds
to 36 seconds with the optimised build.
Merge pull request #75 from adamreeve/poroelastic_static_extension_fix

Fix failing static extension example for coupled elasticity and fluid pressure