matplotlib and pandas as optional dependencies

[maxscheurer]

To reduce the dependencies of adcc, maybe it would be good to remove matplotlib and pandas from the dependencies. They are only required for specialty features (plot_spectrum and to_dataframe) but carry a long list of dependencies themselves, even if they're not needed for "core" features.

The correct versions can be "enforced" in conda recipes by run_constrained and also for pypi.

Suggested by @loriab.

[maxscheurer]

Closed in #141.