Construction of tables for the group scheme, pair interactions and
dispersion correction are now separated. The resulting tables are
never re-used for something else. This uses slightly more memory, but
makes the logic rather more simple. Some of the tables are now held by
reference by their owners, rather than by value, which might improve
cache locality a little.

With this change, we can implement the table support for the Verlet
scheme without getting involved with the group-scheme code, and will
have an easier time removing the group scheme.

Refs #1666, #1852

Change-Id: I8ca608f0e41b02723e6080b80b04d9e7ff048900