Add flags for only reporting one characteristic of the final beam

[zmitchell]

Maybe you only care about the intensity, or maybe you're going to feed the output of polsim to another program and you don't want to deal with parsing the output to select just the intensity. In that case you should be able to invoke polsim like this:

$ polsim --intensity my-simulation.toml
0.75

These are the flags that I think would be useful to implement:

• --intensity
• --x_mag
• --x_phase
• --y_mag
• --y_phase
• --ellipse-angle
• --ellipse-major-axis
• --ellipse-minor-axis
• --eccentricity

Some ideas for useful tests:

• Is it possible to print the full set of results even when one of these flags is present?
• Does the output when the flag is present normalize properly when the -n/--normalize flag is present?