Treat VectorizedInterpolation as a true vector interpolation where possible

[KnutAM]

In #1010 errors are thrown when trying to use a VectorizedInterpolation as a "true" vector interpolation (which doesn't rely on a scalar interpolation in the background).

In many cases, there are some significant performance advantages of using that there is a scalar interoplation that has been vectorized (e.g. for the L2Projector as this reduces the mass matrix by factor vdim).

But this also leads to special-casing in the code-base, so it would be nice to explore where we could get rid of the specialization to simplify the code without inflicting significant performance degradations.