Welch-Satterwhaite equation

File name : 'WelchSatterthwaite.m'. This file can be used
for calculating the degrees of freedom for a
Student's t-distribution according to the Welch-Satterthwaite equation
Four input values : 'std1', 'st2', 'n1', 'n2'
One output value: 'df'
std1: Standard deviation of group 1
std2: Stanard deviation of group 2
n1: Number of arguments group 1
n2: Number of arguments group 2
df : Degrees of freedom
Developed by Joris Meurs BASc (2016)