Expanded explanation of Sigma Electronegativity Descriptor

Change-Id: Ic37241a6a3d161f873b3d2290c41be7c86ab99c3
Signed-off-by: John May <john.wilkinsonmay@gmail.com>
Now also has a task to create a source distrib that includes the unit tests (per user request)

Change-Id: I6e47a18d2084ce5cf83efc731e2b3db95acbb6b5