Allow more detailed specification of mass lumping

The mass lumping can now be switched on/off separately for
(1) the Schur complement substitution
(2) the pressure operator
(3) the multigrid preconditioner for the pressure operator
Note that mass lumping is always ignored at lowest order.
Added detailed xml class information

Added detailed output of different solver options in XML format to the individual classes. The main program now writes out a file solver.xml which contains detailed information on the used solver and can be used to check exactly which options were used in the solution process.
Merge branch 'master' into more_finegrained_lumping_options