Implement Toast operator for demod mapmaker

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 #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 #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.