Create plot_schedule_altitude() function

Just as suggested on issue #362, please add a sister function to plot_schedule_airmass() to be able to plot a schedule which only takes altitude as a constraint. Also plot_altitude() has "Airmass" not "Altitude" on the y axis.

Start by reading the existing plot_schedule_airmass() and plot_altitude() functions. Add the altitude-only schedule plotting function requested in issue #362 and ensure plot_altitude() labels its y axis "Altitude" rather than "Airmass"; verify the plotting behavior and label after the change.

Written by the indexing model from the issue text.