Common Constellation operations

[aburrell]

Is your feature request related to a problem? Please describe.
When I try to use a Constellation, I find it to be not very useful, because there's not common time index and I can't specify a custom function that operates on the Constellation instruments as a group. At the moment, it's basically just a list of Instruments.

Describe the solution you'd like

• Common time index for a Constellation
• Constellation-level custom functions that use data from one Instrument to operate on another, and so on.
• empty property at constellation level
• const_to_inst method that will output the constellation data in an Instrument object.

Describe alternatives you've considered
Just not use Constellation objects

Additional context
Constellations are not as robustly tested as Instruments in a workflow context, due to their development history. These are just the first two things that I thought should exist.