Allow user to specify where source property/element is

[aajacobs]

Currently, Smart Mastering requires sources to be in a specific location in the envelope (see https://marklogic-community.github.io/smart-mastering-core/how-does-it-work). While this can still be the default, users should be able to override the location of their source information, similar to how timestamp locations are declared, so they don't have to change their data model to use Smart Mastering.