Correct nviter and convergence_parameter reporting in output

[timothy-nunn]

Description

PyVMCON interface in PROCESS now updates the nviter and convergence_parameter values so they can be correctly reported in the OUT.DAT and MFILE.DAT

I have also corrected the dummy variables in the _solver_callback because they were the wrong way around.

And, I have removed nviter from the regression test comparison because it does not make much sense to fail if the number of iterations has changed (we should only care about the solution).

[timothy-nunn]

Large tokamak now reports the below in the OUT.DAT:

VMCON error flag                                                         (ifail)                           1     
 Number of iteration variables                                            (nvar)                           45     
 Number of constraints (total)                                            (neqns+nineqns)                  26     
 Optimisation switch                                                      (ioptimz)                         1     
 Figure of merit switch                                                   (minmax)                          1     
 Square root of the sum of squares of the constraint residuals            (sqsumsq)                 1.871E-04  OP 
 VMCON convergence parameter                                              (convergence_parameter)   4.087E-09  OP 
 Normalised objective function                                            (norm_objf)               1.600E+00  OP 
 Number of VMCON iterations                                               (nviter)                          8  OP 

The MFILE also agrees