Add computation of light crossing time of a cell.

[ffoucart]

Proposed changes

Add function to approximate the minimum crossing time across a cell, when moving at the speed of light.
The exact use of that function in the MC code will depend on how we provide GZ information to the algorithm.

Upgrade instructions

NA

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.

[kidder]

you can squash the changes

[kidder]

dx_inertial[d] -> gsl::at(dx_inertial, d)

This will do bounds checking in DEBUG builds