Add limit to time step of packet in high opacity region

[ffoucart]

Proposed changes

Implement limit to the time step that a packet can take if close to a cell boundary in a high opacity region (to deal more accurately with regions where only packets very close to a cell boundary can escape that cell).
This improves the accuracy of e.g. the optically thick sphere test for large optical depths.

Upgrade instructions

None

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.

[ffoucart]

Well, the auto-merge feature didn't go great... let me resolve the conflict with develop...