gh-208: new configuration format

[ntessore]

Implements a new "configuration" format using YAML, which is really more of a serialisation of the data that Heracles needs to run.

Quoting from the provided example file:

# Although we call this `configuration', it could be more correctly
# characterised as `serialization' of the data that Heracles needs to run, in
# the form of a YAML file.  The entire configuration is free-form and returned
# as a namespace by `heracles.load_config()`.  However, certain sections such
# as `catalogs` and `fields` are understood and used by the Heracles CLI.

The motivation behind this design is that we have struggled for a while to condense Heracles' structure into a true "descriptive" configuration format. This solution here goes fully the other way, providing a 1:1 serialisation of the data that would be used in a notebook or script. In other words, this configuration format is basically equivalent to Python code snippets with the same content.

The result has advantages and disadvantages, which we are surely going to discover going forward. One advantage is that it becomes very easy and flexible to load configuration from a file and use it in notebooks or scripts:

# load configuration
config = heracles.load_config("experimenting.yaml")

# use catalogues and fields as configured
catalogs = config.catalogs
fields = config.fields