Support more than one dosing rate based on the chemical key

[fletchapin]

Is your feature request related to a problem? Please describe.
Dosing rate supports a dictionary based on chemical key in PyPES, but the GUI currently supports only a single dosing rate per node.

Describe the solution you'd like
There should be the option to specify the chemical and add multiple dosing rates (e.g., coagulant and flocculant).

Describe alternatives you've considered
This is a real issue because you'd have to edit the PyPES model outside of the GUI to add more dosing rates at the moment.