Crop the training waveforms to a fixed length prior to writing the file for multiple population analysis #569

In PR 554, we have added a feature where we can create multiple populations for training/validation/testing at the same time. For training and validation, we crop these waveforms at the time of training. We would like to crop the waveforms to the same length at the time of generation rather than at training. This will help us save some memory space and training time, as the precomputed waveforms are cropped in advance.