Calculate chi squared over dof

[mantid-roman]

#11617

To test run this script:

ws = Load('MUSR00015189')
func = 'name=ExpDecay'
a = Fit(func,'ws_1',StartX=0, EndX = 20,Output='out')

func = 'name=ExpDecay,Height=61.5684,Lifetime=2.0672'
b = CalculateChiSquared(func,'ws_1',StartX=0, EndX = 20)
print a[1], b[3]

The printed out numbers should be approximately equal.