Implements a point source feature.
Source terms can be implemented as point sources in the problem file. A point source is a new class that stores point coordinates and a values PrimaryVariable vector with a source value for each equation. Point sources can also be solution dependent. For that the user adds a point source at the wished location with values 0.0. Then in the method solDependentPointSource() one gets all point sources associated with the DOF that the method is called for and can add to their values, values dependent on the current solution. The user specifies the source in kg/s (very convenient!).