Relented on the strict import of direct objects/classes for the typing
module only. Typing module components can be directly imported because
it makes the code really painful to read and write otherwise.
This is still in-line with the python style guide adopted from Google at
http://google.github.io/styleguide/pyguide.html section 2.2.
Refactored the location of the progress_callback so that the UI can
specify it and ensure it is used by the automagics process as well as
the plugins themselves.
The progress_callback function has also had an optional description parameter
added, so that it can differentiated in case two scans happen
simultaneously.
- moved segment loading out of __init__(), now happens on first use
- fixed mapping() to return the correct chunk_size
- self._base_layer is a string, not a layer
Mike Auty