fixed deprecation warning from pandas

[Iximiel]

Description

I am working on some (small) modifications on the python package, while running the tests I found that the python module uses delim_whitespace that is deprecated from pandas2.20, (see the manual)

I used the suggested sep="\s+" option, which should be backward-compatible also with older pandas versions

Target release

I would like my code to appear in release 2.11 (or before?)

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.

[GiovanniBussi]

@Iximiel could you perhaps backport this to v2.9? If it's a sort of LTS, I guess we should fix these small issues as they appear, at least when it's easy (as it is now)