Store the best solution vector achieved during the run

Previously the best solution vector achieved during the run was stored, and this was output instead of the last solution, with a suitable warning (#602). Note that this was the vector with the lowest convergence parameter, not the one with the lowest constraint residuals.

I think this was removed when the PyVMCON was introduced. It would be nice to get it back.

Proposed solution

As close as possible to what we had before as in #602, Option 2. The terminal output for an unconverged run would look like this:

==>   11  vmcon iterations. Normalised FoM =   1.942  Residuals (sqsumsq) = 4.5E-05  Convergence param = 1.0E-04
Best solution vector will be output. Convergence parameter =  7.752E-05

If the single convergence parameter is no longer available, we could just output the solution with the lowest constraint residuals.
The PROCESS models should be run again with this solution vector so the OUT.DAT and MFILE can be created.
Comments @timothy-nunn?

Start by tracing how PyVMCON stores and selects solution vectors, then compare the previous behavior described in #602, Option 2. Check how the selected vector is used to rerun PROCESS models and create OUT.DAT and MFILE; done means an unconverged run reports the selected convergence or residual criterion and outputs the corresponding rerun results.