Add sigma-classical function to estimate lower bounds

[GuiMacielPereira]

The lower bounds of the ncp fit can be calculated by the "sigma-classical" function that Matthew has on Matlab.
This improvement would mean that users only need to fill in the upper bound of the fitting parameters, as the lower bounds would be automatically calculated by this function.

The only extra input required is the temperature at which the sample was experimented on.

Contact Matthew for more details.