Tests : Should ensure at least one physics test case with links in boundary conditions

[David Dickinson]

Whilst we have several test cases using kt_grids='box' and boundary_option='linked', on a quick skim it looks like these are primarily setup such that there are no links (jtwist > nakx). This means we are not really testing code associated with linked boundary condtions.

We should introduce a linear test case actively using linked boundaries with > 0 links and we may also want to introduce a nonlinear test case in this regime.