Some code improvements

[NicolasCARPi]

First commit is just formatting (with black --skip-string-normalization tools/eln2md.py) and using /usr/bin/env for the shebang.

Second commit is:

• use constants for filenames
• use isort for import statements formatting
• use Pathlib instead of os
• fix bug where content was appended to previous file if no templates were found
• remove extra newline at end of generated file

[NicolasCARPi]

Next would be use proper arguments parsing with a positional argument for folder path (defaulting to current dir), and a --format option, along with quiet and verbose modes.