Proposing: compacting lower and upper walls into a single file

[Iximiel]

Description

In the context of the WALLS, I compacted Uwalls and Lwalls into a single file (without the update of the documentation from #1111, but with the code correction)

The idea is to remove the repetition (there are only 17 different lines in two files of 158 and 157 lines, which should work in a mirrored way) and get to debug\update a single file.
Then I would like to port something similar also for the non-scalar version of the walls since I think it could bring advantages (no jit and compile-time optimizations and less cache misses).
And since the code itself would be the very same maybe compact also the non-scalar variant into a single one.

What do you think?

This is a constexpr solution, so to back-port it will lose some of it "slimness".

Target release

I would like my code to appear in release ???

Type of contribution

• changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
• changes to a module not authored by you
• new module contribution or edit of a module authored by you

Copyright

• I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

• the module I added or modified contains a COPYRIGHT file with the correct license information. Code should be released under an open source license. I also used the command cd src && ./header.sh mymodulename in order to make sure the headers of the module are correct.

Tests

• I added a new regtest or modified an existing regtest to validate my changes.
• I verified that all regtests are passed successfully on GitHub Actions.