Stow

Changed
- __contains__ logic changed to check against manager so that if edits have happened behind the scenes that they are updated in the manager

Fixes
- amazon isdir couldn't verify to level directories due to a naming issue which has been resolved

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Added to the stateless interface and corrected issues with artefact interface + manager interface

Added
- isabs, abspath, relpath, commonprefix, commonpath to stateless interface
- isabs added as abstract method to managers
- isEmpty added to Directories

Changed
- Put method now requires a strategy for putting an artefact onto a directory
- Get method throws ArtefactNotFound error when get source isn't found instead of ArtefactNotMember
- join doesn't use relpath - it joins and cleans the input

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Added to artefacts properties to extract information about their paths a little easier. This will help massively with cutting down the code a user would have to write to filter/analysis a directory.

Added
- `basename` and `name` added to artefact - for directories this does the same thing - for files, name doesn't include the extension of the file
- `extension` added to File to give a clean way of getting the file extension.
- `Manager` not raised to init so that it can be accessed easier for extending managers

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