Incar writing refactoring

Checklist

• I have read the contribution guidelines.
• Unit tests have been added for any new or changed code, and the test suite passes. Note: Your request will likely not be merged without the appropriate tests.
• "closes #XXXX" is in the body of the MR description to link the related issue, if applicable.

Summary

• Refactoring the write_incar method of ase.calculators.vasp.create_input.GenerateVaspInput.
• Based on the INCAR writer from @martin-schlipf (see !2588) which handles most of the formatting and actual file writing.
• Pulled out a couple of functions (check_ichain, set_magmom, set_ldau, test_nelect_charge_compitability) which should make it much easier to transfer this to new calculators.
• Removed support for the net_charge keyword argument which was deprecated.
• Added some tests.

Issues to be solved

1. @stly's tests in ase.test.caclulator.vasp.test_vasp_incar.py now fail. They did work before 455ed83b. The reason they fail is that the Mock objects checked if ase.calculators.vasp.create_input.open is called exactly once. It is not called anymore, instead ase.io.vasp_parsers.incar_writer.write_incar is called. I tried to fix this, but I do not understand Mock enough to do so in reasonable time. I would appreciate if @stly can look at this.
2. There might be the need for more tests. The only thing I spotted was an issue with ichain, and I added a test for that.

For some more context and the overall goal of this MR please see #1340!