correct sorting procedure by using np.copy(); for evaluation
of simplified case remove 'beta_q'; for evaluation of general case
correct the calculation of 'beta';
simplified the general case R_0 != R_90 (remove case distinctions); not
necessary as principle angle 'alpha_sig1' now lies within +/- pi/2;
apply mechanic book formula with switched sign and check positive
z-rotation;
clean up (remove 'check_consistency_alpha')
remove 'equal_stress_characteristics'