`thickening_parameter = 1.0` is not possible

[MSallermann]

This can currently not be allowed because of the way we compute the final lava flow thickness as

(H_final - H_initial) / (1.0 - thickening_parameter)

In Mr. Lava Loba this works, since they track the thickness differently. For now, I have simply disallowed thickening_parameter=1.0 in the input validation (27b22f8).

[MSallermann]

If thickening_parameter=1.0, the topography does essentially not get modified so there might be some overlap with topo_mod_flag here. See also #13.