Fix #60: "strand sometimes no conserved when AvoidPattern is localized"

[Zulko]

This fixes #60 (@veghp @deto). Changes:

• The fix: In overlap_region(self), the final location always gets the strand of self.
• Cosmetic: I renamed variable names (to left_location and right_location) in overlap_region because 5-year-ago me wrote this method in a very confusing way.
• Added a new test based on @deto's example in Issue with AvoidPattern (and potentially other builtin_specifications) #60. The test fails before this MR, succeeds after.
• Formatting: The commit is a bit polluted by a couple of auto-edits from running black to format the files (sorry for that), but didn't want to undo the changes as they go in the right direction.

I didn't version-bump because I'm not 100% sure what the workflow is there @veghp

[veghp]

This is a duplicate line -- see line 43 -- just double checking if you had something else in mind.

[Zulko]

@veghp I belive this can be merged and released, closing #60 .

[veghp]

Thanks, I merged with dev -- Travis seems to be down, so I'll release it once it's okay. Also addressed #53 to be released with it.