Transit flows cost and spatial aspects

[kndehaan]

The modelling of costs and spatial impact of transit flows has been improved, covering:

• Costs of required import facilities of energy carriers, added to the cost export and to the detailed annual cost chart
• Spatial requirements of required import facilities of energy carriers, presented in a new table of future spatial requirements

Goes with:

• https://github.com/quintel/etsource/pull/3029
• https://github.com/quintel/documentation/pull/179
• https://github.com/quintel/etmodel/pull/4236
• https://github.com/quintel/etengine/pull/1416
• https://github.com/quintel/etdataset/pull/996