Handle case of single dtopo time for instantaneous uplift

Proposed fix for problem when a single time is specified in dtopo file and dtdtopo(m) = 0. in this case.  New feature allowed in 5.2.1 and used in update to geoclaw/apps/tsunami/chile2010, but breaks that example as @mjberger found when running with debug flags turned on.

Also added some comments on what's being done in this part of the code.