Refine adapter guidelines

[MakisH]

• Clarifies R.2 and A.11 for integrated adapters.
• Clarifies in A.4 that logging level can be configurable at compile time.
• Clarifies A.10 to make it easier to answer it.

Takes into account reviews of:

• OpenDiHu adapter: Add OpenDiHu adapter checklist #56
• DuMux adapter: Add reviewing checklist for DuMux adapter #57
• G+Smo adapter: Add G+Smo adapter standardization #52

[carme-hp]

in A4, does it make sense to have as a requirement the option "no logging"? I would expect only "debug" and "release" to be required.

[MakisH]

in A4, does it make sense to have as a requirement the option "no logging"? I would expect only "debug" and "release" to be required.

The original motivation was that one might want to completely switch off the adapter-related information (only), since they consider the adapter only a small/technical part of the overall simulation. In that sense, it would make sense to not get additional log lines for every solver time step, for example. While this could already be covered by a release level, a no logging could additionally disable any configuration/initialization info, for example.

This was originally suggested by @uekerman. This is now an additional requirement anyway. I will add a suggestion.