Revise handling of package metadata and build-system

This MR addresses some housekeeping on the package-level:

1. Switch from a pure setup.py build to a PEP517/518 compatible system using setup.cfg and pyproject.toml
2. Set package-wide defaults for flake8 and pytest
3. Add a gitignore file

moist_thermodynamics/__init__.py

 from .functions import *
 from . import constants
+
+
+__version__ = "0.5.0"