[md][dualnetwork][couplingmanager] use templates, add comments, use DimLessNumbers

What this MR does:

This MR should cleanup the couplingmanager for the dual network a little bit. Before, there were a lot of functions duplicated just with other domain indices (one function for void domain and one for solid domain). Now templates are used and those functions for each domain are merged.

At the same time, I used the DimensionLessNumbers from Dumux to calculate the reynolds number. This include is quite nice as other dimensionless numbers such as Prandtl or upscaled Nusselt number can be also calculated with this at a later stage. However, it can be argued if this include is necessary. I think it's nice to use, what was already implemented before.

As well a few comments have been added.

There is now corresponding issue open.

Notes for the reviewer

Before you request a review from someone, make sure to revise the following points:

• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)
• is the code you changed and/or the new code you wrote covered in the test suite? (if not, extend the existing tests or write new ones)
• does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in CHANGELOG.md.
• is the list of the header includes complete? ("include what you use")
• all files have to end with a \n character. Make sure there is no \ No newline at end of file comment in "Changes" of this MR.