Add a function to compute the average calibration in utils.jl

[pasq-cat]

i would like to try to add a function that plot the average level of calibration in utils.jl.

the function is defined in https://arxiv.org/pdf/1807.00263

[pasq-cat]

the end results should look like this for a miscalibrated network

and like this for a calibrated network

[pat-alt]

Yes, nice idea! Have a look at https://github.com/devmotion/CalibrationErrors.jl for reference. It would be nice to have the basic evaluation function here, but the plotting in TaijaPlotting