Update nodal district heat share and impose bounds on heat source utilization

[amos-schledorn]

Current behaviour

National district heat demand is resolved based on ratio of urban/rural population in node. This creates problems when introducing heat sources, such as geothermal heat (#1359), as non-transmittable heat sources are aggregated across relatively large model regions.

Expected behaviour

1. Resolve district heat share based on source data. This improves spatial matching between heat sources and district heat demand.
2. Impose bounds on heat source usage (e.g. bounding by district heat demand close to source) or modify the clustering to district-heating-system archetypes per node rather than the current aggregation.