Enable OpenMP in isf Poisson solver (SGFFT) to increase performance

Description

The isf poisson solver is already parallelized using openmp, but it has not been used since 2011. Now it is enabled and makes the OpenMP version faster when using more cores.

Note: the MPI version of the isf solver is faster on one core than the serial version because it uses a different algorithm.

News snippet

Enable OpenMP in isf Poisson solver to increase performance

Checklist

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