Split collecting subnetwork demand into internal sources and main network phases

[SouthEndMusic]

Fixes #1207.

[SouthEndMusic]

To do:

• Add an @assert within allocate! to make sure that for the main network only OptimizationType.allocate is used;
• Add set_initial_demands_level! and adjust_demands_level! analogous to those for user demand (this eventually also has to be done for flow demand);
• Make sure the optimization types are properly handled in all functions called within allocate!;
• Make sure when using OptimizationType.collect_demands, the used demands within the subnetwork are those reduced by what is allocated for OptimizationType.internal_sources;
• Update docstrings;
• Update documentation
• Add tests for reduced subnetwork demands when the subnetworks have internal sources with a modified version of the main_network_with_subnetworks model

[SouthEndMusic]

For an explanation of why I removed testing the last value of this vector see #1327.