Error handling: there must only be one saved match

Coder: throw ex if there is more than one saved match for a given input
Error handling: constraints added to db

DB Helper: contraints added to prevent duplicate data  and nulls
Error handling: catch  any GeoCoder init errors
Code Tidy: Weight  column removed from DB

Weight column not now needed as there is only ever one saved match for a
given input.
DB Helper: weight column removed
Match Provider: weight column removed
Matched Names: Default probability added
Level1Match: Weight removed
Level2Match: Weight removed
Level3Match: Weight removed
DbIntegration Tests: Weight removed
Code tidy: comments updated
Code Tidy: File Import scope reduced to internal
Copyright and assembly version info added