Add check for .popper.yml integrity (#237)
Add check for .popper.yml integrity (#237)

The read_config function in utils.py is modified to check if the
.popper.yml file is not empty and if the "metadata" and "pipelines"
dictionaries are defined in it and fail with a proper message if
either criterion is not met.

Fixes #235.
systemslab/popper,systemslab/popper