Change handling of source terms in a many-levels domain

[rthedin]

The source terms get read before they are interpolated onto all the different levels (heights) of the new grid. In the reading routine, it computes all the levels present in the grid. That is usually not a problems because we do simple grids with flat boundaries, resulting only in a handful of levels (which was even recently improved by PR #26).

The problem, however, arises when using any complex geometry as the bottom boundary, resulting in LOTS of different levels. The solver hangs in the computation of those levels, during zPlanes_.initialize(), which triggers zPlanes_(mesh_). I let the solver run for over a day and it did not get past that call.