initial implementation of linetrace

[alpha-beta-soup]

What this PR does

cf #11 for supporting linestrings. This is a work-in-progress and needs some deliberation.

I tried to come up with an equivalent name to "polyfill", and came up with "linetrace". Happy to change it.

There's a number of ways this could be implemented, depending on a few inter-related criteria:

• Should order be retained?
• Should there be repeating cells (in sequence) in the linetrace output?
• Should there be repeated cells (at all) in the linetrace output?
• Are parts of multilinestrings relevant?
• Should there be an equivalent to explode?
• What should explode do with a multilinestring? (Perhaps a MultiIndex?)

In general I think that the return type should be an ordered list, not a set. However I don't permit repeated cells if they are in sequence. Non-sequential repeats are possible to handle cases like self-intersection.

This currently supports multilinestring input, but information about the parts is not retained. I thought about an extra parameter to handle these, maybe retain_parts, True by default.

Happy to take direction on the appropriate behaviour, but at some level there's probably an element of opinion, unless there's a standard (even a de facto one) for how linestrings should be represented in a DGGS---if the idea makes sense at all.

[alpha-beta-soup]

Both explode and index_parts are now supported. Behaviour is hopefully clear through tests.

@DahnJ I'd appreciate a review; at this point I've implemented everything I care about for this feature.

[DahnJ]

Hi @alpha-beta-soup thanks so much for the contribution! Sorry for my late reply on my side, I have been busy with other things.

I think the PR looks great, thanks for providing the tests and submitting such clean code in general!

With regards to your design choices, I think they all make sense. Since you're the first to request this feature, I would hold off from implementing anything like retain_parts unless someone requests it.

Just had a comment about index_parts, otherwise this already looks great ⭐

[DahnJ]

Looks great now, thanks a lot for the contribution @alpha-beta-soup !