Sfftk-rw

* bugfix for using `SubtomogramAverage` class in `SFFShapePrimitiveList` class

* added `SFFSubtomogramAverage` class

* In EMDB-SFF `v0.8.0.dev1` `SFFSegmentation.merge_annotation(other_seg, include_colour=False)` (*do not* merge colour by default)

add Python3.10 support

* removed use of `os` package for exit codes (problems on Windows)
* fixed errors in `developing` documentation

bugfix: reading from HDF5 retains some ids as `numpy.int64`

* tests added and pass

add Python3.9 support; dropped Python2.7

* add official support for Python3.9
* dropped support for Python2.7 and Python3.5
* now in beta but stable
* new version: v0.7.0