Make full screen optional

[JohnGriffiths]

Following idea from @pellet - this is a tiny PR to make the use of full screen optional.

This is particularly useful for debugging, because it allows to both see and switch to a terminal while the psychopy window is up, and allows to retain control via the terminal upon a crash, for e.g. to view the stack trace or enter a debugger.

This is done by providing a use_fullscr Bool argument in the experiment init, and setting it to a modifiable attribute of the Experiment object.

A modifiable attribute is also added for window_size, although this is not added as an input argument upon init.

I think it would probably be a good idea to move some other parameters such as the monitor type into modifiable object attributes in the future also.

Example usage:

from eegnb.experiments.visual_n170.n170 import VisualN170
expt = VisualN170(duration=10, use_fullscr = False)
exp.window_size = [1200,500]
exp.run()