Better handling of catch trials

I removed the catch trials from the conditions. This means now every trial can potentially become a catch trial. Whenever the randomly drawn nFramesPre > nFramesPreMax, the trial is treated as a catch trial. I don't think we have to change anything on the LabView code, but please test it before actually merging and closing this pull request.

We do have to adjust the parameters in the config file:
* Remove nCatchPerBlock; the fraction of catch trials is now determined by P(nFramesPre > nFramesPreMax), which is controlled by the parameters of the exponential distribution (nFramesPreMin/Mean/Max)
* The block size is now sum(biases, 1) * numel(coherences) * nRepsPerBlock, i.e. nCatchPerBlock less than previously. We should adjust the subBlockSize accordingly to be an integer fraction of this number