Reintroduce the 121 filter #184
Description
The 1-2-1 filter has been turned off because of issues it caused with the chunks and potentially the sigma-sigma depth interpolation. For the chunks, this was because a corner or diagonal that was split into chunks would not filter across the chunks and so would result in a different solution. For the sigma-sigma depths levels, the depth levels will be varying so averaging along them would also mean averaging over depth with could cause static instability.
To reintroduce the filter, it could be turned into a function that is run once the chunks are assembled together. The id_121 will need to be re-calculated for all points instead of chunks.