lattice_primitive: add 6D primitive lattice

These are needed for the force unit test, as the size of stress tensor
will grow to a 6-element tensor in 3D

Fix typo in comments.
forces/pair_full: (re) introduce compute() and compute_aux()

These functions simplify this force module and make its implementation
similar to pair_trunc again
host/forces: make stress_pot_type public

In the gpu force modules these are public and for consistency make them
public in the host variants, too
Calculate the full symmetric stress tensor

Previously only the trace and diagonal elements were stored limiting the
use of the stress tensor to isotropic situations.

In order to allow coalesced access of the stress tensor in the gpu
implementation, the matrix elements are stored in column-major order.
This, on the other hand, creates the need for special reading and
writing functions for conversation between column- and row-major orders.