[RM-9208] [powerLaw] add positive constraint on amplitude and min/max constraint on power coeff

Created on 2024-09-17 15:11:05 by Didier Vibert. % Done: 0

positive constraint can be hard coded.
min-max constraints on power coeff must be parameterized (and maybe authorized an inifinite value for the param ?)

apply a very crude constraint optimisation:

• positive: if negative, set to zero and NAN for the power coeff
• min/max power: if outside range, recompute amplitude and chi2 with power coeff fixed at boundary