[pointsource] Major improvements on pointsource interface

* Implement problem interface functions mimicking the normal source interface
* Implement a possibility to implement derived point source classes getting cutomized lambda functions
	* the pointsource class gets an update method
        * An example implementation with test for time dependent point sources is implemented
* Restructure the point source class. Do not derive from PrimaryVariables (less felxible) instead overload
  some operators for convenience
* Call the solDependentPointSource function for each point source not per dof (multiple ps possible)
* Recompute the map for adapted grids