Code structure improvement, initialize and apply code linting

[ahennr]

In particular, this PR includes:

• addition of a .gitignore (TBD: do we want to track changes in input, predictions and output folder ?)
• introduce flake8 and isort and its configuration in setup.py
• split dependencies to base and dev dependencies (folder requirements)
  • Installation of dev requirements using pip install -r requirements/development.txt (conda environment must be activated first)
  • Installation of base requirements using pip install -r requirements/base.txt (conda environment must be activated first)

Affects #2

Please review @Momen-Mawad