Reset depths of major discontinuities.

The depths are set to match those existing in the input velocity model. The initial values are set such that if there is no discontinuity within the top 100 km then the Moho is set to 0.0. Similarly, if there are no discontinuities at all then the CMB is set to the radius of the planet. Similarly for the IOCB, except it must be a fluid to solid boundary and deeper than 100 km to avoid problems with shallower fluid layers, e.g., oceans.