Simple benchmarks

This is based on test-improvements branch (#529) and must not be merged before.
Only the last four commits contribute to this PR.

Few benchmarks using the GroundwaterFlowProcess are added here.
For different mesh sizes 1 to 1e6 a square [0,1]^2 and a cube [0,1]^3 are prepared as VTU mesh input.
Dirichlet boundary condition is imposed on left (1) and right (-1) sides of the domains.
The expected benchmarks' results are projections of analytical solutions of the Laplace steady-state problems wrt the above BCs on the different meshes.

All test files are placed in external data dir.
Questions are welcome.