Add WCS pointing support to ground simulation example

[tskisner]

• Add 3 possible pixelization schemes to the workflow, with a helper function to select between them.

• Cleanup a few places where the pixel distribution was created.

[tskisner]

In order to test these changes, I did the simple exercise of creating a focalplane:

toast_fake_focalplane --min_pix 7 --fov_deg 5

And then created a schedule:

toast_ground_schedule \
    --site-name "Atacama" \
    --telescope "LAT" \
    --site-lon "-67.8" \
    --site-lat "-23.0" \
    --site-alt 5200 \
    --patch "small_patch,1,40,-40,44,-44" \
    --start "2020-01-01 00:00:00" \
    --stop "2020-01-01 06:00:00" \
    --out schedule.txt

And then tested the workflow with all defaults:

toast_sim_ground.py \
    --focalplane focalplane.h5 \
    --schedule schedule.txt

I then also tested using the WCS pixelization with all defaults:

toast_sim_ground.py \
    --focalplane focalplane.h5 \
    --schedule schedule.txt \
    --pixels_healpix_radec.disable \
    --pixels_wcs_radec.enable