FIX-2.9 Now the forces from LWALLS should be correct

[Iximiel]

Description

This is about #1091, now if a lower wall is asked with an odd exponent, it should give the correct force, and I think also the energy because ene += k * power; now will contribute with a positive and repulsive term in any case.

I think we need some more tests to check this.

Target release

I would like my code to appear in release v2.9

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.

[carlocamilloni]

shouldn't we back port it as back as possible?

[Iximiel]

shouldn't we back port it as back as possible?

I agree

[Iximiel]

The new commit changes the documentation: I noted that the offset for UPPER was added, and for LOWER was removed, so I slightly changed the documentation accordingly, this should also reflect that the wall is repulsive with the absolute value before the power, (it is correct to put the absolute value in the documentation like that?)