Slow to add demand to the network

[trevorb1]

Feature Request

Adding demand to the network is quite slow. While fine for the electrical network (since there is only one load to add) sector studies add 10+ different demands. Im pretty sure its just the reading in of the data from the CSV files, not the actual adding of the data to the network.

Suggested Solution

As implemented in PR #465, pickle files are used to speed up the IO. May be worth exploring a similar solution here as well.

Additional Info

No response