HSC backport: allow for use of Approximate model in background estimation

This issue involves transferring changesets from the following HSC issues:

• HSC-145 Investigate approximating rather than interpolating backgrounds
• HSC-1047 Background object cannot be loaded with butler
• HSC-1213 Set background 'approximate' control settings when background control is created.
• HSC-1221 tests failing in ip_diffim
• HSC-1217 Verify backgroundList IO works properly when Approximate is enabled in background control - HSC JIRA

The Approximate (Chebyshev) approach greatly improves the background subtraction around bright objects compared with the interpolation scheme currently in use (which over-subtracts near bright objects).