Fix bug in demand_share module

[tsmbland]

Description

Only spotted this because of the refactor in #631

There was a mistake in the new_consumption function. It was passing current market data to unmet_demand. However, this should be calculating unmet demand by existing assets in the investment year (not the current year). There are still things I don't understand about this function, but this seems obviously wrong so needs to be fixed.

All of the results have changed, but the changes seems small and everything still looks reasonable to me. The only model that looks a bit different is the carbon budget tutorial. Not sure about this, but will probably just have to play around with that model to see what's going on. I can't see anything incorrect about the changes in this PR

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass: $ python -m pytest
• The documentation builds and looks OK: $ python -m sphinx -b html docs docs/build

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works