Federated, multi-datacenter support

[chad-earthscope]

In the future more data centers will be offering a (hopefully standardized) data availability service. This may also require adapting ROVER to a slightly different availability service. Using the IRIS Federator, ROVER could collect data from multiple data centers.

This is a placeholder and should not be acted on until the availability interface is standardized and implemented at multiple centers.

[chad-earthscope]

fdsnws-availability has been standardized: http://www.fdsn.org/webservices/
ROVER has been modified to use the DMC's implementation.