Assert that field names have to valid Python attributes

[VeckoTheGecko]

This enables us to consistently use attribute access such as fieldset.U

• use assert_valid_python_varname function already in particle.py (move out to _core.utils.common)
• Add call to this function in VectorField, and Field init, and into Fieldset.add_constant. Try and except, catching and rewriting the error message
• Add tests

[nilodna]

Assigning myself. I'm planning to tackle this one in the coming days

[VeckoTheGecko]

Awesome! This should be quite straight forward. You can use _assert_valid_python_varname

• adjusting the error message that's raised to something like Received invalid Python variable name {name!r}
• Moving the helper function to _core.utils.string so that its not in particle.py

and of course by adding a test as needed

[nilodna]

Hi @VeckoTheGecko! I think I’ve covered all the main changes in the code (VectorField, Field, FieldSet.add_constant). The tests are a bit tricky for me right now, though. I’ll open a draft PR so you can take a look at what I’ve got so far. The testing part might take a little longer to finish.