Support searching on fields of models other than Tree

[RickMohr]

Searching on any field of a map feature subclass (e.g. plot.length, rainBarrel.capacity) gives an error like The model name must be one of the following: mapFeature, polygonalMapFeature, tree, species, mapFeaturePhoto, udf. Not plot.

We need to handle both built-in fields and custom fields.

For custom fields we can't just search on mapFeature. If I create a "color" custom field for both bioswales and rain gardens, searching on mapFeature.color would find both bioswales and rain gardens.