improve interoperability for PyCBC in external waveform packages

[ahnitz]

PyCBC has as one of its key goals the idea of providing a simple interface for GW waveform generation which can be used by humans, but also higher level algorithms such as searches and parameter estimation.

To help aid with this, we've added a plugin architecture so that external waveform packages can advertise and export a compatible interface, allowing PyCBC to recognize its capabilities and automatically be able to use its waveforms if the package is installed.

We should consider adding support in the following external packages (some such may have explicit support already, but probably should move to plugin model).

• gwsurrogate (advertise gwsurrogate models to pycbc sxs-collaboration/gwsurrogate#29)
• teobresum
• lalsuite
• seobnre
• python imrphenom

[yi-fan-wang]

A possible error related to waveform plugin #3750

[ahnitz]

A PR has been submitted for gwsurrogate to add integration sxs-collaboration/gwsurrogate#29