FitResults interface

[tBuLi]

The interface for getting value's, stdev and covariance from a FitResults instance should be changed to the following for readability:

(Let a, b be instances of Parameter)

fit_result = fit.execute() # Any FitResults instance

print fit_result.value(a)
>>> 3.0

print fit_result.stdev(a)
>>> 0.5

print fit_result.covariance(a, b)
>>> 0.0

print fit_result.variance(a) 
>>> 0.25