Absolute delta for phase

Phase is a relative thing measured in degrees. 0.1 degree and 1 degree has delta of 0.9 degree and it's meaningless to say that the second phase is 10 times higher.

1. Change the Cavity class to show phase difference in degrees, not in %.
2. Implement "wrapping around" property of phase: 179 deg and -170 deg have difference of 11 degrees (and not 349 deg)