Enable real time changes to task parameter GUI values

[HenryJFlynn]

Background:

In the MATLAB protocol. task parameters can be changed throughout the duration of the experiment. This is not the case for the PyBpod protocol. The task parameter GUI pop ups at the start of an experiment and disappears once the user hits OK. The parameters are locked in for the rest of the experiment

Goal:
Enable the user to change the task parameters in real time throughout the duration of the experiment, similar to the functionalities in MATLAB

See HenryJFlynn/mouse2afc#31 for the original issue