Ground simulation improvements

[keskitalo]

This PR

• improves the --alternate mode in the scheduler to work with --elevations.
• adds support for elevation-modulated scans:
  • raster scan : step boresight up or down after each scan pair
  • sine wave : oscillate the elevation during scan along a sine wave
  • triangle wave : scan at constant rate in elevation.

[tskisner]

These changes are fine. After the toast-3 release we could consider making a new "schedule" sub-package and move some of this work into that rather than the monolithic commandline script. Thanks for getting the updates to the TODGround class in before that work is ported to toast3.