Added a provenance field to VariableStatement objects

[enoriega]

This field will let us know which pipeline/software component generated the extractions by attaching a new data class to the variable statements

[enoriega]

@Tranway1 Please check the new field to indicate provenance on VariableStatements

I took the liberty to fill in the value on the MIT converter, but please double check it to make sure it looks okay for you.

Please let me know what you think

[Tranway1]

This change makes sense to me