rewrite GROMACS easyblock to build all four variations in the same build

[akesandgren]

single precision without MPI, single precision with MPI,
double precision without MPI, double precision with MPI.

Change the default for double_precision from False to None
And a bunch of other changes.

This supersedes #1987

[Flamefire]

General:

• Needs rebase on master, especially as CMakeMake supports build_type and build_shared_libs
• Always check for the longest possible string when checking for a config option. So -DFoo=bar not DFoo=bar just to avoid false positives, the extra dash doesn't hurt but makes it safer, even though only slightly
• Check for uses of self.cfg['foo'] where foo is from extra_options. @boegel insisted that self.cfg.get(foo, default) is used so deriving classes not calling the base extra_options don't make this throw a KeyError. However as this EB is unlikely to be derived from, I'd say this is OK especially here (I personally prefer the KeyError in general). Check with @boegel
• Replace self.dynamic by self.cfg['build_shared_libs'], see note
• Handle CUDA vs double in 1 place, e.g. build_all_steps. BTW: Is this a limitation of GROMACS? CUDA generally can do double just fine. Maybe this doesn't apply anymore?

I'm not to confident on the multi-step builds, so comments are mostly general and CMake specific.

[akesandgren]

Attacking the comments from @Flamefire piece by piece...

[akesandgren]

Reg "Replace self.dynamic by self.cfg['build_shared_libs']" perhaps later. The dynamic flag comes from the cray toolchain and self.dynamic been hijacked a bit and I need to be able to enforce it to be on for later versions.

I can look at that later.

[akesandgren]

Reg "Handle CUDA vs double in 1 place, e.g. build_all_steps", haven't look at what i can do in that.

Can look again later, leaving it alone right now.

[boegel]

Retested extensively with a whole range of easyconfigs, old and new, works like a charm.

Thanks a lot @akesandgren!