Issues fixed

Issue 765: [mfront] Fix numerical tests in the Swift and Power hardening rules

For more details, see https://github.com/thelfer/tfel/issues/765.

Issue 705: [mfront] fix the computation of the derivative of some isotropic hardening rules

The corrected hardening rules did not take into account a theta factor in the expression of the derivative of the elastic limit, which could affect convergence when this parameter differs from 1.

For more details, see https://github.com/thelfer/tfel/issues/705.