The name "getFirstDerivative" is more appropriate, since "grad" would refer to spatial derivative, which is not the case here (this function computes the
derivative of the damage shape function with respect to the level set field).

The xEvalGradTLSDamageFunction (renamed into xEvalTLSDamageFirstDerivativeFunction) was modified accordingly, as well as the test cases in
/test/damageShapeCurves.