use_local_tz for plot_altitude

`plot_altitude` is currently very similar to `plot_airmass` but it is missing the possibility to use local timezone.
I don't particularly mind using `plot_airmass`, but that is the only reason I am doing it.
I think it would make sense to have it available in both.

Locate the plot_altitude and plot_airmass entry points and compare how timezone handling is exposed. Check the surrounding plotting tests or examples before making the change. Done means plot_altitude offers the same local-timezone option as plot_airmass without changing existing behavior.

Written by the indexing model from the issue text.