Ensure positive phase advance

[austin-hoover]

The calculation of the phase advance in MATRIX_Lattice.trackTwissData is:

delta_phi = math.atan(mt.get(ind0, ind1) / (beta_0 * mt.get(ind0, ind0) - alpha_0 * mt.get(ind0, ind1)))

If the phase advance is greater than $pi / 2$, delta_phi will be negative and we need to add $\pi$ to make it positive. We could use atan2 but I just added an if statement below.