allow for using any fields for lat/lng

[jronallo]

I was hoping to implement a new map interface using blacklight-maps to replace this interface: http://d.lib.ncsu.edu/collections/map
I already have a lat and a lng field stored in Solr and would like to use those fields instead of reindexing everything with new fields. This appears to not be possible at this time.

[mejackreed]

There is a lat/lng branch that is just not merged into master that may accomodate this. I will take a look at it and see if it will work.

https://github.com/sul-dlss/blacklight-maps/tree/to-latlng