fix: avoid error when scaling from scenario with slightly negative generation

[danielolsen]

Purpose

Avoid an AssertionError when calling AbstractStrategyManager.populate_targets_with_resources() using a ScenarioInfo object based on a Scenario where prev_generation for a state/gentype evaluates to a slightly negative number. This can be seen when trying this on Scenario 409.

What is the code doing

Adding a small tolerance range to the negativity check in set_generation(), where if the value being passed is negative but within this tolerance, it is set to 0. Also adding more description to the error message on the assert.

Time to review

Extremely short, it's 5 lines of changes. It can be tested using Scenario 409: it should fail using develop but pass using this branch.

[BainanXia]

This implies that we may have some negative values in pg within the tolerance range (-1e-3~0) from the optimization results?

[danielolsen]

Yes! The joy of the barrier method, it's fast but it returns a solution that is not actually at a vertex, merely within a given tolerance range of a vertex. For Scenario 409, the assertion complains about a value on the order of -3e-5.