Merge branch 'feature/pointsources-rebased' into 'master'

Feature/pointsources rebased

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!).

* [x] Move point source calculation from implicit problem to a helper function
* [x] Make PointSource a Dumux property so people can use cutomized point source classes derived from a base class
* [x] Consider extrusion for dim < 3 when calculating the volume

* [x] Implement TimeDependentPointSource?
* [x] Make it work for adaptive grids
* [x] Implement PointSource constructor without value (for solDependentSources), the value is then set in solDependentPointSources.

This branch has been rebased recently onto master.

See merge request !36