ps: functions for combining dispersions/means (#332)

* add `band` option to `write_block` and background writers

* compute and save amplitude dispersion from real SLCs

* start adding truncate, use ps to calc

* repack the amp dispersion rasters for better compression

* add `truncate_mantissa` option to `repack` functions

* fix test, set upsampled boundary to nan in compress

* Add functions in `ps` to combine amplitude dispersions

These can take older rasters, assuming the `N` was saved, and produce a new mean and dispersion

* add make the truncate into `round`

* remove `Example` from docstring

* update the changelog for this release

* [skip ci] reformat to be in style of https://keepachangelog.com/en/1.0.0/

* remove commented `N` shape check

* add latex documentation

* remove helper function for var combination

* fix amp mean function name change