Introduce inclusion_mapping operation

[schnellerhase]

Given two meshes $T_\text{from}$, $T_\text{to}$ that are contained in one another $T_\text{from} \subset T_\text{to}$, in the sense of vertices existing in both. The inclusion_mapping function computes the global indices in $T_\text{to}$ of the 'vertices in $T_\text{from}$.

The computed index mapping thus generally describes how one refined mesh is contained in another and is a first step for a multi grid transfer operator.

This is a strictly local computation. If a vertex is not to be found in $T_\text{to}$ is is marked as $-1$ in the map.

Calling inclusion_mapping with a pair $(T_\text{coarse}, T_\text{fine})$, where $T_\text{fine}$ is produced by refinement of $T_\text{coarse}$ and using the identity partitioner, it is guaranteed, that all locally owned vertices will be found in $T_\text{fine}$.