AbstractFunctionValues

[KnutAM]

Adding this here as a reminder and in case the discussion here enters the Slack hole.

To be considered after #764 and #798

Currently, there are PointValues and PointValuesInternal which allow updating the quadrature point location. PointValues just contains a CellValues whose quadrature points are updated during reinit!, and PointValuesInternal has only the shape values in the reference domain (which for identity mapping are the same as in the spatial domain).

If we introduce AbstractFunctionValues or another way to have multiple variants of FunctionValues (see #764), which each have a different number of derivatives, we can use these to construct AbstractValues which are able to give only the value (current PointValuesInternal only), value and 1st derivative (current CellValues, FaceValues, PointValues), or value + 1st and 2nd order derivatives (currently not implemented, but requested).
(This would also merge PointValuesInternal into PointValues and make it possible to expose this API for use in e.g. FerriteViz.jl)

[KnutAM]

The merged FunctionValues has the DiffOrder type parameter and PointValues and PointValuesInternal are merged. The API for requesting a different DiffOrder is not public/documented yet, since we might want to change the API after introducing the option of adding DiffOrder higher than 1.

With that generalization, I think this issue can be closed. If AbstractFunctionValues comes up again but for a different reason, that would be a separate issue.