Move cuboid remapping to ngc_selection script and update

[maho3]

This PR moves the code in cmass/survey/remap_cuboid.py into a helper function within cmass/survey/ngc_selection.py, for the reasons discussed in #40.

Changes:

• Cuboid remapping is moved to cmass.survey.ngc_selection
• cmass.bias.rho_to_halo now feeds cubic volumes directly into cmass.bias.apply_hod
• bugfix in cmass.nbody simulators, such that they now save overdensities instead of raw densities
• default cosmology is now Planck18 instead of Planck15
• ngc_selection's rotate function is replaced with move_to_footprint which applies a better rotation/translation operation to more evenly center the box on the survey footprint
• rho_to_halo's halo velocity interpolation has been updated (and bugfixed) to now call theory velocities when CIC velocities are NaN
• ngc_selection's reweight function now exactly matches the n(z), and warns when it doesn't have enough galaxies.
• ngc_selection now has a random_rotate_translate option which allows you to augment the dataset by randomly moving the box

All of the above has been validated in matts_tests/test_remove_cuboid.py

[maho3]

Here are some plots showing that the new move_to_footprint centers the simulation box over the survey footprint.

Simulation cuboid wrapping the survey volume, when it has been transformed to the origin.

Simulation box overlapping with the survey footprint in comoving space (without transformations).