Add growth seed parameter

[tsmbland]

Description

This introduces a new GrowthSeed parameter designed to prevent the MaxCapacityGrowth constraint from being too restrictive when existing capacity is low/zero.

The previous approach was to remove the MaxCapacityGrowth constraint when it would otherwise be zero, but this still doesn't help when capacity is small but nonzero. I've frequently seen users putting tiny values in their existing capacity files (e.g. 0.000001) and wonder why that technology isn't invested in - it's because MaxCapacityGrowth, even if big in percentage terms, will cap absolute growth at a tiny number.

Thus the need for a seed, which relaxes the constraint when existing capacity is low.

The main changes are in the max_capacity_expansion function in constraints.py, and explained in the documentation. I've discussed with Adam and he's happy with the new method in principle. I'll also make sure to explain this in the release notes as it's quite a fundamental change which will likely affect most models.

I've re-run all the models and regenerated the results files. Rather than adding a GrowthSeed column to all the technodata files, I'm just relying on the default seed value of 1 which seems reasonable for these models. Most of the changes to the results are small and don't affect the narrative of the tutorials. A couple of models didn't run at first so I had to make a couple of small parameter tweaks in the default_adhoc and trade models.

Also, I couldn't get the modify-time-framework to run due to issue #371. As described in that issue, the model has a bug where gas capacity sometimes sporadically drops to zero. When this happens, new capacity must be invested in to make up the difference, and the default seed is too restrictive in this case. I could toggle with the model to make it happy, but since it's broken anyway I've decided to mark this particular test as xfail until #371 is fixed

Fixes #578

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

[dalonsoa]

Looks good to me!