Store computed distances

[markusbaumeister]

Currently, distances between vertices (or faces) are not stored.

Implement optional storage in the following fashion:

1. Add a record component DistanceMatrix to the representation of twisted polygonal complexes.
   This component should store partial distance information about the vertices.
2. Add the methods "StoreDistances" and "DeleteDistances" to initialise the matrix or to delete it
   completely.
3. Within each distance calculation, do the following:
   -> Check whether distances are stored (by checking whether the matrix exists)
   -> If it exists, check whether the result is known
   -> For known result, return it
   -> Otherwise: Compute the distance
   -> If distances are stored, the result is written in the matrix