Calculate and store WTG parameter

[nichollsh]

We model these planets as a 1D column. However, this cannot easily capture dynamical processes such as Coriolis effects and equatorial jets, which would redistribute heat from the substellar point elsewhere.

This could be partially handled by #241.

However, we can first estimate how reasonable this 1D column approach is by calculating the weak temperature gradient (WTG) parameter (see [1] and [2]). This can be easily calculated and stored in the helpfile. It would allow estimates of the validity of the single-column assumption for different planets, parameters, and points in time.

This does require the user to provide the planet's rotation rate in the configuration file. In theory this rotation rate would evolve over time due to tides, but we can keep it fixed for now. This can be easily done by allowing the user to choose between various day-orbit resonance ratios (e.g. 1:1 for a synchronous rotator) or by inputting a day length directly.

1: https://doi.org/10.1146/annurev-fluid-010518-040516
2: https://doi.org/10.1098/rspa.2016.0107