Plug PETSc (and various classic solvers and preconditioners)

The 'LinearSolver' class proposes mechanism to choose the used
- library,
- method, and
- preconditioner
Also one can adjust the required convergence epsilon, the maximum
number of iterations and the verbosity.

All of these options can be listed through the 'pgs' script using the
'printLSAvailable()' builtin function.

One drive the linear system solver by changing values in the
script. For instance:
	setLSVerbosity(true);
	setLSEpsilon(1E-9);
	setLSMaxIter(1000);
	setLSLibrary("PETSc");
	setLSMethod("LU");
	setLSPrecond("none");

Finally one can print the current options by using the
'printLSOptions()' function.