#251 Remove half-initialized state in DvrOperator and TransformationOperator

[Ulf Lorenz]

Both TransformationOperator and DvrOperator have a special internal constructor that is used only by derived classes. These special constructors leave the operator in a half-finished state, and the actual operator class then fills the grid.

That is a strange pattern. Preferably, the specific DVR or FBR potential should be given through a function object as parameter.