Resolve Issue #22, #23, #24, #25

[hyeeyoungkim]

This pull request includes:

• Resolve #22 by adding additional description to bhl_inventory/README.md
• Resolve #23 by updating column names for check_missing_folder.py, check_output_structure.py, make_dips.py to match bhl_inventory.csv from 2019-02-19
• Resolve #24 by adding additional usage instructions to README.md
• Resolve #25 by adding logic to remove separations from target_list in make_dips.py