Sfftk

* new `mergemask` option `--skip-assessment` to go straight to merging and skip checking binary-ness
* reinstated `--allow-overlap` flag; overlapping masks blocked by default; failure on the first detected overlap
* fixed a bug which excluded some data from merged mrc headers
* updated documentation to outline best practices when merging masks (only use merging for relatively small volumes
otherwise convert large binary masks into STL meshes)
* fixed bug when deleting fewer than the total number of external references
* new tests to check mask merging

* `sff prep mergemask` to merge multiple binary masks
* updated documentation to reflect new feature

* not all search features remain e.g. `--start` is ignored, search results count unavailable

* viewing transforms
* inserting transforms from `.map` files
* modifying transforms using the `notes` utility
* fix typo in parser