dependance on 'when2heat'

[brynpickering]

The When2Heat dataset is a heat demand dataset that can be found on OPSD. We use some of their scripts to produce heat demand. We don't depend entirely on their scripts, since they can only handle national shapefiles and are limited in scope. To get these scripts, the script repository is cloned as a rule. This should be updated to do one of the following:

a. clone a specific commit hash, to ensure some degree of reproducibility
b. re-implement the scripts locally (I think the licence is sufficiently permissive to allow this)
c. request/submit upstream changes to When2Heat so that it works for all our requirements, rather than only a subset, and can be installed via pip/conda-forge.