Wip/first UI

[luigibrancati]

Added a first UI with the following features:

• Homepage with country selection. This also allows the user to select multiple countries either with the multiselect (top box) or using polygons on the map. The sidebar should show up to 3 countries and a country count after the 4th
• Page for GHG yearly plot
• Page for EAC4 anomalies plot
• Page for both Hovmoeller latitude and Hovmoeller Pressure/Model level plots. The plotting api isn't quite there yet, but this page will allow the user to select one the these 3 plots types

I also added a CLI that starts the streamlit app. To use it, install the package with pip install . and run atmospheric-explorer on a the terminal.

Small changes to other modules.

[elisaliv]

General note: are there also more complex ways to manage state in Streamlit? Or patterns like separating state management in different files? I have looked a bit online in the documentation, but it doesn't seem so.

Another general question/idea: we could add all state variable default values to a config file (or similar). Maybe not right now but during the final refactoring and refining phase.

[luigibrancati]

@elisaliv I answered you comment and added Issue #18, but will reply here too.

As far as I can tell, Streamlit only provides the session_state dict-like to keep track of app state, this dict is shared between refreshes and also between different app pages. Why would we want to separate session between pages? Things like the selected_country are used by all app pages after all.
We could define a wrapper around st.session_state, but I'm afraid that would mostly over-complicate things.

About the session state in a config file, I can't think of a way to do it. Config files are used to define things, not to initialize them:

• If we use a config.py file and put code to initialize the session state, we must import it in every module and will overwrite the session state with every import
• If we use a config.cfg or YAML or JSON for what matters, we'll have to import it, fetch the keys we need on every page and initialize them, i.e. exactly what we do here with just one more step