WIP: Migrate local algorithms

• Conceptualize extensions
• Write extensions archetypes
• Write extensions concepts unit tests
• Write extension concepts documentation
• Write ndimage::clamp/periodize/buffer
• Update documentation
• Write border manager (method + policy)
• Write boder manager user documentation
• Write border manager unit tests
• Think & design compound extension for the future (usefulness) : border builder with flags (FILL_LEFT, IMAGE_RIGHT for tiling ?)
• Migrate local algorithms to using the new border manager
• Write unit tests for algorithms
• Write algorithm documentation (for user)
• Write canvas documentation (for dev)

fixes #46

requires !57 (merged) !62 (merged)