bugfix: pass down trial parameter to subclass.

[pellet]

bugfix: pass down trial parameter to subclass as was previously done before VR code modified rendering logic.

Also fixed baseclass 'present_stimulus' method signature to match arguments as expected for subclass to implement.

This was something which I accidentally had modified the behaviour of - I had trouble understanding what the 'trial' parameter was used for in the experiments.
It seems like none of the example experiments are using the parameter.

[JohnGriffiths]

@pellet - summary from discussion:

looks like the trial variable actually wasn't ever being used

could you please first check that in the rest of the code and confirm

if so, update this PR to remove that variable entirely as it's redundant

thanks