Minor updates to PlotTrajectories

[nilsvu]

Proposed changes

• Plot a marker at the current position of the black holes.

Also some improvements to help with calling the plotting functions from outside the CLI:

• Downsample data outside of the plotting function. Don't downsample by default.
• Allow to pass in a figure object in which to plot.

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