Give a name to the porosity

Mostly one would assume that this would be the porosity since that's the
only non-named argument, but stating this explicitly saves us from one
trip to the manual.
Use a more realistic injection rate

In addition, this also shows how to use the units to specify the
measure.
Sink should match source

Although it already does numerically, this make is more explicitly
clear.
Use natural names instead of mathematical notation
Merge pull request #269 from rolk/269_sink

Use a more realistic measure for source and show use of units