`RawContainer` time shift issues

[ljgray]

There are a couple of underlying issues with changing the .time property of RawContainer to centre time samples:

1. The .time property is not perfectly compatible with draco containers - example, applying a draco RFIMask to a CHIMETimeStream fails because the .time properties do not line up. This is because the underlying index_map is not changed, so the first entry in the index_map["time"] array is different than the first entry returned by .time. I could fix this by making some modifications to ch_pipeline RFIMask and adding a slight wrapper around draco RFIMask to handle the times correctly, but there is the more underlying issue of this incompatibility when dealing with any time-series containers.
2. CollateProducts copies the time axis directly from the .time property, rather than the index_map. This results in:
   • An additional shift in the time axis such that it has actually been shifted by a full sample
   • A change in the way the index_map["time"] property is indexed, causing this implementation to fail. This is accounted for in the draco .time property implementation.

All of these issues couldbe fixed by implementing a task to directly shift the index_map time, rather than adding the new .time property.