prioritize snapping points in AutoGrid

[dmarek-flex]

I found the snapping_points idea very useful, but also that the default value for the min_step was too restrictive. Here I propose some changes that give priority to the snapping points over the computed interval boundaries.

1. I changed the behavior so that when dl_min is defined, it becomes the size used as the min_step. This is needed for Simulations with PECs since the calculated max_dl is much too large to be useful.
2. If the snapping_point is close to an existing boundary it will replace it. The logic I used should still result in intervals that satisfy the min_step requirement though.

[weiliangjin2021]

This is great, thanks! Just add a test and CHANGELOG and good to merge.

[dmarek-flex]

This is great, thanks! Just add a test and CHANGELOG and good to merge.

Done

[dmarek-flex]

@tylerflex @weiliangjin2021 Any last comments?