WIP: encapsulate operations in the volvars update method

What this MR does / why does DuMux need it:

This merge request should remove duplicated code often seen in the volvars' update functions. This issue is outlined in #814.

Notes for the reviewer @tufan, @nedc, @hanchuan, @yue

**Question: ** does it make sense to add new headers to the dumux/material/constraintsolvers/ directory, or would another location make more sense?

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)
• does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in CHANGELOG.md.
• 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.