Build helper functions

[michaelbornholdt]

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.

[michaelbornholdt]

@jccaicedo just wanted to check in here. I think the functions exist in the branch and they are still usefull.
Maybe we mention them in the Wiki and have them on a side branch? Not sure where to store them.

The checking for black crops is crucial. The other one not so much.