add user-defined maxwarn options to all steps (EM, MD_relaxed, MD_NPT)

[iorga]

Currently, maxwarn option can be set up only for FEP (see #113). We need that user can change it in all steps (via the runinput.yml file), so this option should also be implemented in EM, MD_relaxed and MD_NPT.

Right now, with Gromacs 2020.3, the EM in octanol is blocked because of a warning:

WARNING 1 [file system.top, line 29]:
  The following macros were defined in the 'define' mdp field with the -D
  prefix, but were not used in the topology:
      FLEXIBLE
...
Fatal error:
Too many warnings (1).
If you are sure all warnings are harmless, use the -maxwarn option.

[VOD555]

MD_relaxed and MD_NPT now support maxwarn innput.
Current EM cannot support maxwarn as the EM function in gromacswrapper doesn't have this input.

[orbeckst]

Can we rewrite the MDPOW code to edit the MDP file when using octanol? Arguably the cleaner approach is to not include -DFLEXIBLE and remove it from the template file when we are setting up the octanol EM.

Raise an issue in GW to allow passing the maxwarn flag in the EM function.