Included the get_built_ratio function

Merged Nicolas Garcia Ospina requested to merge add-built_ratio into master

This function takes a quadkey and source_id and compares the estimated built area with the one associated to OBM. If it is not found it returns 0, otherwise it returns the ratio.

I want to use this function to produce the built-ratio field that later would lead to a completeness assessment.

I would be grateful with comments on how to make it better or if there is a better option.

\approve @prehn @fd @ds

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