Create plot_schedule_altitude() function

[jkampars]

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.

[wtgee]

Hi @jkampars, thanks for opening the issue. Any chance that you want to try writing this up and submitting a PR? It's a good first issue and first PR for helping to contribute! ;) Otherwise there are unfortunately no guarantees on when this can get done as everyone works as a volunteer. Hopefully you'll consider giving it a try! Cheers!