add a simple builder for elasticity data

[computron]

This would be to move data from the elasticity collection to the materials collection so that elasticity could be one of the materials summary properties. It should be a very simple builder to write - just matching the task_id to the materials_id and then copying over the most relevant information.

See:
https://groups.google.com/forum/#!topic/atomate/zaPBgt03wTU

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