Draft: Add a function to return component vapor pressure in 1p adapter

What this MR does / why does DuMux need it: In some calculations, vapor pressure of the component is needed, e.g., converting relative humidtiy to mole fraction in the free flow channel.

Notes for the reviewer

TODO: insert text here

Before you request a review from someone, make sure to revise the following points:

• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)
• is the code you changed and/or the new code you wrote covered in the test suite? (if not, extend the existing tests or write new ones)
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• is the list of the header includes complete? ("include what you use")
• all files have to end with a \n character. Make sure there is no \ No newline at end of file comment in "Changes" of this MR.
• (if not applicable remove) are newly introduced or modified physical values/functions backed up with a scientific reference (including doi) in the docs?
• (if not applicable remove) if the examples are modified, is the documentation regenerated (using generate_example_docs.py)