Called in TauPTime. Computes a new tau model for a source at depth using the previously computed branches for a surface source. No change is needed to the branches above and below the branch containing the depth, except for the addition of a slowness sample. The branch containing the source depth is split into 2 branches, and up going branch and a downgoing branch. Additionally, the slowness at the source depth must be sampled exactly as it is an extremal point for each of these branches. Cf. [Buland1983], page 1290.