Implement Toast operator for demod mapmaker

[tskisner]

This issue is to capture the details discussed in other venues. Here is the overall plan:

• Refactor some of the demod mapmaker code to have several subroutines that can be used either in the site pipeline or in the toast operator. This is being implemented in Sat mapmaking refactor #973 .
• Extract general toast.Observation to AxisManager translation code from the MLMapmaker operator into a common place. Ensure that any metadata from the original load and preprocess stage is propagated back from the observation metadata into the AxisManagers.
• Create a new wrapper operator for the demod mapmaker. This should call helper functions from Sat mapmaking refactor #973 to instantiate the correct type of mapmaker being used. It should then perform any post-demodulation processing by running a separate PreProcPipe on the data. Then there will be a loop over observations, calling add_obs() to accumulate map domain products before writing the outputs in the operator finalize() method.