Improve the storage of MG vertex dofs.

Improve the storage of MG vertex dofs.

The DoF indices for a vertex are stored in an array where we just
collate the indices for each level. There is currently another
array that stors the offset within this array where the DoFs
for a given level start. This array is dynamically allocated,
but it is altogether unnecessary because the offsets are computable:
they are simply the number of the multigrid level times
dofs_per_vertex.

Consequently, get rid of the array and replace it by storing dofs_per_vertex.
We can then easily compute the starting offset wherever necessary, rather than
having to look it up.