Convergence with sin(x) proved.

- new computation of L2 norm using XFEValues
Solving convergence of XFEM and SGFEM.
Improve exact solution class, nodal convergence.

- it is possible to set frequency a amplitude of sin
- better constructors
- better organization in main
- first attempt for nodal convergence
Miscellaneous changes.
Enrichment simple boundary condition.
Fix bug in mapping the well onto unit cell when it is not there.
Improve code readability, better print output.
Method for setting known values of dofs.
Add test method for squares of adaptive intergration, clear main.