Add Transition to Ringdown Script

[AlexCarpenter46]

Proposed changes

This is the first iteration of the transition to ringdown script for the BBH's. To use this script, you have to have a BBH that has reached common horizon and output reduction and volume data at the time of common horizon finds using the pipeline, then you can do something like

spectre bbh start-ringdown path/to/final/segment/directory/from/inspiral --path-to-output-h5 Ringdown_Coefs.h5 --match-time 5000 --number-of-steps 10 --settling-timescale 10.0 -L 2 -P 9 -O Ringdown_9 --no-submit

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

This script is still very experimental, if you try it out and things don't work or break, please let me know :)

[knelli2]

Please add fixups for each commit since this is a pretty long PR

[knelli2]

We can leave this for now, but ideally in the future I think we'll want to get rid of this

[knelli2]

Looks good. Squash