Get rid of the quadrature points inside the well.
Improve analytical solution - sigma considered.
Add adaptive integration test, add residuum computation.

- xmodel - refinement_level made static constant
- residuum computation in xmodel
Add xfem_ramp template in XFEValues.
Numeric constant parameters gathered, xfem_ramp template use.
Documentation - ESCO and others.
TODO and comments.
Add features to Model - integrate difference.
Move system staff to Model_base, create output_options.
Use output option flags as introduces before.