New tutorial to calculate radial velocity from multiple measurements

[alexbinks]

This tutorial is designed to assess the multiplicity status of a
star and calculate a "final" radial velocity for stars when there
are several (> 2) individual measurements. Whilst intended for
RV measurements, the method can be applied for any
parameter that needs to be weighted from different sources.
Principally the program:

1. reads in a table of RV measurements for any set of stars
2. makes a decision on whether the star is multiple or likely single
3. calculates a final RV from the individual measurements.

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[hamogu]

Thanks for contributing! This looks interesting, but I think you might want to see if the indexing logic can be simplified a bit. At least I don't understand it - but maybe that's just because I don't know that much about the topic.

[kelle]

This looks really promising! Thanks so much for the contribution and we'll work on giving it a robust review ASAP. I'm also going to start a thread in the #learn channel on slack.