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Adds toPointString method to Point model

This outputs a string in the appropriate format for a VoxelMap .points file
entry.
Implements API#buildFile

buildFile assumes the request body is a JSON array of string point ids. For
each point specified in this array, it adds its point file entry to a string,
which is Cached for a brief time using a simple hash as a cache key.

This hash is returned in the response body for use in downloading the points
file.