Patched the rational non simplified switching function around X==R_0

[Iximiel]

Description

First of all, sorry for the Friday afternoon PR.

As we discussed in #1038, here I patched the numerical instabilities around R0 in the non simplified rational switching function. And I used the test from #1038 for ensuring the correctness of the change (v2.8 won't pass the new test).

Before merging: should I update the changelog?

Target release

I would like my code to appear in release v2.8

Type of contribution

• [x] 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

• [x] 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

• [x] 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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.90%. Comparing base (221032c) to head (9ccf5b9).

:exclamation: Current head 9ccf5b9 differs from pull request most recent head 8768f05. Consider uploading reports for the commit 8768f05 to get more accurate results

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v2.8 #1048 +/- ## ========================================== + Coverage 85.87% 85.90% +0.03% ========================================== Files 595 595 Lines 49748 49752 +4 ========================================== + Hits 42720 42741 +21 + Misses 7028 7011 -17 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[GiovanniBussi]

Great! I see that other tests are not affected, I am afraid there was no test on the 2 N!= M case, or maybe no test was probing that point?

I would add something in the change log for version 2.8

Thanks!

[Iximiel]

I am afraid there was no test on the 2 N!= M case, or maybe no test was probing that point?

I think that was the reason it slipped trough the tests. We have to thank intel that approximates differently than gcc

I update the changelog, I think this can be squashed+merged