Create a class to associate a record in one source with a record in another.

[MoralCode]

This would need to at least store the Source object (as defined by models/source.py) and the identifier for the individual record being accessed for both the records from the source and target that are deemed to be a match

This class must also:

• preserve the order of the data sets. For example, whichever dataset is specified by the user as the "source" (see alllow the source and target datasets to be specified by name using argparse arguments #16 ) should be the "source" here too.
• Be able to handle cases where only one of the two sets of source+record ID are present (such as the case when there is no record that is an adequate match)

This class should also have the ability to store a confidence level for how certain the match is and should be able to handle associations with only one source defined. the operation of #12 will determine if this is needed