A few restrictions apply:
* The bc->value is interpreted as an inflow *into* the connecting
cell. Outflow conditions must be specified using a negative
sign.
* We do not currently support general total flux conditions
(necessitates the introduction of additional degrees of freedom),
so bc->cond_pos[i+1]-bc->cond_pos[i] must be one (1) for all
BC_FLUX_TOTVOL conditions.