Better handling of carrier resolution

[brynpickering]

For instance, at the moment it is possible to have 'water_heat' and 'space_heat' as separate carriers or aggregated into one carrier ('heat'). The workflow currently works with all, but will break on anything other than 'heat' when filling templates in the euro-calliope submodule. This breaking change was introduced in one of the two latest commits, to push through a rapid update to how heat techs are represented.

Anyway, the method for handling these different carriers is quite messy and could do with cleaning in general.