calculate barycentric velocities with barycorrpy

[megbedell]

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This isn't strictly necessary because we have to read the HARPS data headers anyway to get the time of observation and airmass, so we might as well take the BERV from there too - but for the sake of independent checks & flexibility, we should be calculating BERV inside wobble. The barycorrpy package seems promising: code & paper

[davidwhogg]

At the very least we should be checking the HARPS headers. I'm not sure I trust barycorpy more than HARPS at this point. Or yet.

[megbedell]

I'm marking this as low priority right now because I think in general the BERVs calculated by the instrument pipeline will be better than what we can get ourselves. This is because the instrumental data reduction should be calculating a BERV using the exposure meter time series to properly weight the effective timestamp of the observation, but we don't have that time series ourselves so we can't reproduce it. As a result I expect that our BERVs would be wrong at the few m/s level.