Config schema

[brynpickering]

We can validate our config using a JSON Schema (see the Snakemake docs), which would help to manage what a user is able to define.

As well as listing configurable options, we can set which ones are required and which are optional (with defaults) and limit the configuration data type and set of possible entries. Already mentioned in #7 is the need to limit the NUTS years that can be defined.

Others I see are:

1. Countries, they must be within the European continent
2. Bounds, they must make sense as coordinates
3. Administrative units, must be in [nuts0, nuts1, gadm0, lau2, etc.]
4. Floats in parameters must be positive, and some have more stringent ranges