Sympy potential

[ludogibbs]

Description of changes:

• new feature: generate non-bonded tabulated potentials from sympy expressions

[RudolfWeeber]

Thanks, Lukas. Plesae address the following:

• Add your test case to the CMakeList.txt in testsuite/python, so it gets actually run in CI.
• translate the comments in your code to English
• pls use a more meaningful arument name than f for the energy expression
• I like the string-based expression input, but it does not cover all cases (e.g. piecewhise functions or other more unusual sympy expressions). So please keep that but also accept a sympy expression in additoin to a string for the argument.
• A furhter argument distance_symbol should be added, so people can use something else than r. Keeping r as default is fine, I guess.

[jngrad]

Thanks! Don't forget to add the test to the CMake build system and to install and run the linters on your workstation. Here are the relevant sections in the developer guide:
Linter and code formatter, Python integration tests.

[RudolfWeeber]

Looks good. Just 2 things:

• please rename the f argument to sth more meaningfull likle energy_expr
• the linter is complaining about the test importing something fro mSympy you aren't using.

[RudolfWeeber]

@jngrad could you pls take a look? I am not sure, what the linter wants.
Once that's resoled, we can go ahead with this pr, form my side.