Two grid with non matching interpolation

[schnellerhase]

See #1

[schnellerhase]

• Change to the use of explicit interpolation operators (so a matrix and its adjoint)
• Use the coarse to fine interpolation for this, as the pointwise nature of the interpolation operation would drop information going the other way around - fine to coarse just drops values on DOFs that are no longer available.
• move the restriction/prolongation routines into cleanly separated functions -> first step to an interface to allow for custom operations

[schnellerhase]

To make this work we need to be able to create the interpolation matrix between the two associated spaces, currently fem.interpolation_matrix does not support different meshes though.