Native map-maker design

[tskisner]

This issue is to track the design of the TOAST mapmaker, which exists as notes in various places.
The requirements are:

• Support for Madam-style baselines and noise filter (which in the limiting case of baseline == one sample, is equivalent to traditional GLS map making)

• Support for NPIPE-style (toast-planck) gain and other templates that may span multiple observations

• Modular design that allows chaining sparse matrix operations without need to process full TOD at a time

• Support for on-the-fly pointing and burst buffer caching

• Extendable to support unforeseen systematics templates

• Must support both precomputed and on-the-fly-evaluated templates

[tskisner]

The native map-maker is now merged as of #321 and working, except for one known performance issue which should be a separate issue.