Mc adaptive packet count

[ffoucart]

Proposed changes

Let the MC code choose adaptively the energy of packets, based on a desired total packet count. This currently allows for adaptivity in time, but does not use higher-energy packets in dense regions (this will be coded later).

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.

Further comments

[ffoucart]

Depends on #6403