This commits allows for flexible choice of either ILU(0) or ILU(n).

The ILU level n is a dynamical parameter given in the parameter file. The default is 0 (as before).
In addition the relaxation parameter has been added to the parameter with the
default preserving the state from before.
Also, the default parameter for use_amg and use_bicgstab in the constructor of
CPRPreconditioner have been removed. This replaces PR #251.