Removed a lot of unused functionality in ISMesh.
Updating the tetrahedra in a SimplexSet instead of the simplices in a simplex_set.
Using get functions instead of lookup_simplex in ISMesh.
Using SimplexSet as container for Simplex boundary and co boundary lists.
Removed unused compact/compressed stuff.
Clean up in get_boundary and get_co_boundary functions.
Moved the dimension variable from the parent Key class to its children classes and made it const static (reduces the space consumption significantly).
Removed find_node, find_edge etc, since lookup_simplex did exactly the same.
Updated the get_nodes functions to return a SimplexSet.
More clean up in getters (this time for getting edge keys).
More clean up in the key getters.
More clean up in getters.
Added getters which have a simplex set as input.
A bit of clean up in split function.
More clean up in getters.
Simplified ISMesh split operator.
Minor changes to tests.