OPES: Making parsing not reliant on NaN

[invemichele]

Description

I added a check to avoid relying on std::isnan for parsing.
This resolves issues with intel compiler see #990

Target release

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

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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4f8280c) 85.91% compared to head (b4ace0e) 85.91%.

❗ Current head b4ace0e differs from pull request most recent head fae2c45. Consider uploading reports for the commit fae2c45 to get more accurate results

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Additional details and impacted files

@@           Coverage Diff           @@
##             v2.8     #992   +/-   ##
=======================================
  Coverage   85.91%   85.91%           
=======================================
  Files         595      595           
  Lines       49731    49731           
=======================================
  Hits        42724    42724           
  Misses       7007     7007           

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