Export single-cells command and splits.

[Arkkienkeli]

This is going to be a pull-request with some updates to training strategies, sampling, augmentations.

[jccaicedo]

• Removing sampling of single cells: change the name of the command => export_single_cells and the associated messages
• Change "single_cell_sample" for "single_cell_set"
• Change the if statements for naming the metadata files for initialization in the cli main function.
• Prioritize making TRAINv1 work.
• Can we parameterize augmentation?

[Arkkienkeli]

This comment will be addressed in another PR:

• Can we parameterize augmentation?

Changes:

1. The command to export single cells: export-sc
2. training_values and validation_values have become training and validation to make it consistent with sample_crop_generator modes.
3. The single-cell metadata now has target and split columns as in the main metadata.
4. The option --sample is now --single-cells
5. . single_cell_sample --> single_cell_set in __main.py__
6. Conditions in __main__.py changed.

This should close issues #278 and #275

[jccaicedo]

It looks good to me!
Thank you for cleaning the code and keeping naming conventions consistent.