Constants to be variable

[GarethCabournDavies]

(please excuse the pun title)

As pycbc is moving to be used by more that the ground-based LVK detectors, we could move to a model where constants can be chosen according to the use-case. For example LISA defines its own constants here

I am not sure whether the different experiments will insist on their own definitions being used for official results.

The discussion of why LISA, the LVK, scipy etc should have different values of the planck constant, or a year in seconds etc. is beyond the scope of this

We may be able to do this using an environment variable, an installation option, or as a scheme in the same way we set arrays.

[ahnitz]

@GarethCabournDavies Note that where they exist all constants should come from the standard python stack and not other sources. The order of operations should be astropy -> scipy -> numpy -> other for any new/modified code. Older code should be updated as possible.

I'm not sure of the use case for allowing the sourcing to be something that could differ, but if there is a case, it would be good to have that discussion here.