Add barycentric velocity calculation utilities

[bwmeyers]

Most of the time this isn't necessary, but it is important when searching long time series, and doing acceleration search, etc.

There two functions should provide the functionality we need. It is available from PRESTO at some level, but it's not easily exposed to the user which can make using the values difficult. At this in this case, we could essentially compute the correction as the beam is formed.

[bwmeyers]

For reference, this is the input we should pass to accelsearch for SMART, and my idea is that we can simply compute this and store it with the "Beam" row in the database for each pointing.