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