Warn on missing inverse relationships

[lgebhardt]

Adds method to the Relationship class to get the inverse_relationship. This supports warning once when the inverse can't be found. This is controllable with a configuration setting and is true by default.

It also adds an option to raise an exception if the inverse can't be found. This is false by default. In this default case if the inverse relationship is missing related resources will not collected or counted.

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[bf4]

I like it. I think the better naming of source relationship -> inverse relationship will drive better behavior