Create subroutine to identify observation clusters

[interplanetarychris]

In preparing for an increased level of automation on making TLEs from observations, it would be useful to have algorithm access to "clusters" of observations.

This is most likely implemented in a python function with scans a list of IODs returned from the database, and groups by:

1. Observations from a single station
2. Observations within (X) time of each other -- X could be hard-coded (less than 15 minutes from the same station), or made variable by the characteristics of the satellite mean motion.

It would be useful for the clusters to be identified by an index, as well as a "average(?)" obs time, which would allow removing older clusters of observations from the current set, until some minimum RMS is achieved, or the number of clusters and their spacing prevents further reduction.