Split segmentation.py into a module

[mvanwyk]

Segmentation code should be refactored into its own module, eg from pyretailscience.segmentation.hml import HMLSegmentation rather than from pyretailscience.analysis.segmentation import HMLSegmentation. This will support a clean structure for upcoming features.

• Move BaseSementation into pyretailscince/segmentation/base.py
• Move ThresholdSegmentation into pyretailscience/segmentation/threshold.py
• Move HMLSegmentation into pyretailscience/segmentation/hml.py
• Move SegTransactionStats into pyretailscience/segmentation/segstats.py
• Move RFMSegmentation into pyretailscience/segmentation/rfm.py
• Delete the ExistingSegmentation class. It was an experiment that just added complexity
• Update all tests and docs to reflect the refactoring
• Add the api stubs for the mkdocs (eg these)
• Update mkdocs and add a segmentation module here as has been done for the analysis modules