Importer attributes to geom transform

[rmarianski]

@jdeolive , @ischneider : Some importer changes that shouldn't change existing behavior. Care to review?

These are the changes that were required to support the workflow where a user was asked what columns should represent a lat/lng in a csv file. They boil down to:

• new transformer to convert two specified fields to a geometry
• modifying the logic that copies feature attributes from source to destination in prep for inline transform
• new url param added by importer before creating data store. This is to let the csv data store know that it shouldn't try to guess the geometry based on name but should just return the attributes as it reads them.

[rmarianski]

@jdeolive Is the above test what you had in mind?

[jdeolive]

Yup, that looks great.

[rmarianski]

@jdeolive I removed the context parameter, and made the csv indirect tests more thorough by checking the attributes on the first feature that got imported. If it's ok, can you merge?