Add a source/sink term.

[niccolotubini]

I develop this task in source_sink_term branch

[niccolotubini]

Now the source/sink term is taken into account via the right hand side vector (rhss).

The next step would be to add a time dependence of the source/sink term.

[niccolotubini]

The source_sink_term is merged with master. Source/sink term is still constant.