Some time ago we had agreed that the conversion from a function to a vector and vice-versa should be handled by a member function of the corresponding function class instead of through a free function. That makes it consistent with the refactored matrix creation, see #125 (closed).
This merge implements the discussed change. Function classes now offer member functions
toVector()
fromVector()
which get used by the PETScVector
and TrilinosVector
classes.