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 inputs.
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.
Previous short test on 20x10 rectangular domain is replaced with the benchmarks on the square domain.