Plotly is not in the requirements

[tjstienstra]

When installing the development version of this repo, I noticed that plotly is currently not in the requirements, while it is being used.

[tjstienstra]

It is possible that one should also add pandas to the requirements. plotly seems to require it, but does not install it automatically?

I got the following error: ImportError: Plotly express requires pandas to be installed.

[moorepants]

pandas and plotly should be optional dependencies only needed for the app to run.

[tjstienstra]

import plotly.express as px is in main.py, so you kind of always need it now. It is used in _plot_bicycle_geometry_plotly and _plot_eigenvalues_vs_speed_plotly

[moorepants]

That needs to be moved out to another file.

[moorepants]

Fix proposed in #100.