- Moved tests dir to root
- Moved icclim package to a new src dir
  The above are to follow the best practice describe in:
  https://packaging.python.org/en/latest/discussions/src-layout-vs-flat-layout/
  From the Python Packaging Authority.

- Update build and release process to avoid using deprecated features
  Now we will use `flit` and `build` tools to build icclim
  releases.

- Remove setup.py in favor of the more modern pyproject.toml

- Remove requirement.txt and requirement-dev.txt thanks to
  pyproject.toml

This will avoid getting some segmentation fault.

Context
-------
Since v1.6.1 of netcdf4 py the version used for netcdf-c
release the GIL. However, netcdf-c is not thread safe so
from times to times threads may mess with each other when
we use dask parallelization to open multiple datasets at
once.