Bugfix for tomo code
Bugfix for tomo code

Set the value of self.best_fidelity to -1 initally, such that that
attribute also exists if MLE is not run. If it doesn't, the tomography
results are not saved to the data file.

DiCarloLab-Delft/PycQED_py3,QudevETH/PycQED_py3,DiCarloLab-Delft/PycQED_py3,DiCarloLab-Delft/PycQED_py3