AWH multidimensional coordinate bias

Tests biasing of the two-dimensional dihedral angle coordinate (phi, psi)
for alanine dipeptide in vacuum. This also test the periodic AWH grid.
Note that this test can be unstable. It compares different bias variables
on the grid which is in this case 2D. As MD is chaotic, it can happen rather
soon that runs with different setup, e.g. number of MPI ranks, visit at least
one different grid point. Therefore this test is over 20 steps.
