Build helper functions

Working with millions of files can be a real problem and lead to a lot (!!!) of lost time. Implementing these helper functions will make sure that one can run the big functions (profile, sample-sc, and train) without any errors that cost days of work.

Here is a list of suggestions for the most helpful test functions:

1. **check images**:
This function simply runs over all lines in the index and checks if the images exist. This prevents the profile command form exiting with an error.

2. **Check sc-metadata.csv**
This function should run over the sc-metadata file and check if all images exist in the right place. This needs to be run prior to training to prevent crashes during training.