Extension of the FCNBase for providing the analytical Gradient of the
function. The user-Gradient is checked at the beginning of the
minimization against the Minuit internal numerical Gradient in order to
spot problems in the analytical Gradient calculation. This can be turned
off by overriding CheckGradient() to make it return "false".
The size of the output Gradient vector must be equal to the size of the
input Parameter vector.
Minuit does a check of the user Gradient at the beginning, if this is not
wanted the method "CheckGradient()" has to be overridden to return
"false".