add option to ignore PLUMED version check for GROMACS

[smoors]

(created using eb --new-pr)

each PLUMED version supports only a small set of GROMACS versions.
however, the patches usually work fine as long as you use the same major version.

for example PLUMED 2.7.2 officially supports GROMACS 2021 but seems to work fine with 2021.3
in that case plumed-patch will issue the following warning:

WARNING: engine gromacs-2021.3 not found, I will search for a close match
WARNING: found gromacs-2021
MD engine: gromacs-2021

[akesandgren]

Yeah, I didn't dare do that by default.

[boegel]

lgtm

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS GROMACS-2021-foss-2020b.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
node3152.skitty.os - Linux centos linux 7.9.2009, x86_64, Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz (skylake_avx512), Python 3.6.8
See https://gist.github.com/f7185750f503c0af98b44bfcb3aac839 for a full test report.