Top and bottom chemicals for partition settler

[siddharthab]

This can help the users specify which chemicals will definitely stay in
one of the phases and the rest will go into the other. Useful when
composing systems and you won't know everything about the composition of
the feed before simulating the complete system.

Maybe a process specification can be an alternative to this.

NOTE: There is a small breaking change in fixing a typo in the argument partion_ids to partition_ids. If you think breakage is too big for your users, I can revert the change.

• Clear contribution

Clearly describe what the contribution is.

• Documentation

If a new feature is added or existing functionality has changed, the documentation
should also be updated.

• Tests

Any new or altered functionality should be tested and all tests must pass.

• Code coverage

The code coverage should not decrease so that not unnecessary or untested code
is added.