Add option to sum pixels by squaring, adding and then sqrt().

Adding amplitudes does not make sense theoretically. Squaring
converts amplitude to power, and adding power makes more sense.
This option makes the display less cloudy and more crisp. Gain
can be turned up more without washing out details in large clouds,
showing more detail.

The old cloudy look looks good in its own way, so this should
remain an option, and not become a permanent change.