Addressing the mass balance issue described in #30.
Use ghost cell volumes defined in HDF file rather (had previously not been leveraging this information).
Matrix solver only includes real, internal cells. Necessary information from boundary cells is integrated on the right hand side of the transport equation, since all boundaries have an associated concentration timeseries.
Identified that some of the issues we were observing were due to violation of the Courant Condition.
Addressing the mass balance issue described in #30.