Sfftk

* changed convert argument `--particles` to `--subtomogram-average`
* required sfftk-rw>=0.8.1 to work

* basic working API
* starreader documentation
* skeleton implementation of star converter
* test files
* main entry point for `starsplit` and `starcrop`
* display progres bar while reading RELION `.star` file
* `prep starsplit`: split a .star file by image name; options to handle heterogeneity
* `prep starcrop`: crop a `.star` file to a certain number of rows
* unit tests for all the above
* `--image-name-field` to specify field with image/tomogram

* use `--include-colour` when merging annotations to include manually edited colours from a source EMDB-SFF file