Cleanup and better organize the session_state variables

[blakeaw]

The session_state stores things like the compartmental model and data imported for PD analysis. I think it would be good to organize these better by introducing section-specific dictionaries such as: st.session_state.compartmental that could store variables for that specific section (in this case, the compartmental PK/PD modeling section.). This would help segment the session_state variables and keep things from getting mixed up across the different app pages/sections.