add custom easyblock for FFTW.MPI + modify FFTW's sanity check step to allow checking only for MPI parts of FFTW installation

[bartoldeman]

Use a flag mpionly=True to signify FFTW.MPI in the sanity check.

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS FFTW-3.3.8-gompi-2019b.eb
• SUCCESS FFTW-3.3.10-GCC-11.3.0.eb
• SUCCESS FFTW-3.3.8-intel-2020b.eb

Build succeeded for 3 out of 3 (3 easyconfigs in total)
node3108.skitty.os - Linux RHEL 8.4, x86_64, Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz, Python 3.6.8
See https://gist.github.com/9bec01207c94b740b591ce505bf50c9e for a full test report.

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS FFTW-3.3.10-GCC-11.3.0.eb
• SUCCESS FFTW-3.3.10-gompi-2021b.eb
• SUCCESS FFTW-3.3.10-iimpi-2021b.eb
• SUCCESS FFTW-3.3.8-gompi-2020b.eb
• SUCCESS FFTW-3.3.9-intel-2021a.eb

Build succeeded for 5 out of 5 (5 easyconfigs in total)
node3502.doduo.os - Linux RHEL 8.4, x86_64, AMD EPYC 7552 48-Core Processor (zen2), Python 3.6.8
See https://gist.github.com/a221bb324ab8309387d8410fbc775001 for a full test report.

[boegel]

lgtm