Optimize extract nodes algorithm

[nirvn]

Description

This PR optimizes the extract nodes algorithm by upgrading it to native C++. I've also added a couple of tags to the algorithm to ease its discoverability (let's not throw people into SAGA for such a simple task 😉)

@nyalldawson , your review would be appreciated.

Checklist

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[nyalldawson]

Looks good!

[nirvn]

Nice speed gain: 20.89sec (python) vs 15.36sec (native c++) for a large-ish polygon dataset resulting in 51,000 points.

[nirvn]

@nyalldawson , as suggested, I'm now using a vertex iterator. For the record, it didn't lead to great speed gains. The same dataset now takes 15.13sec, virtually no gain here.

[nirvn]

@m-kuhn , any idea why the tests fail with precision?

[nyalldawson]

any idea why the tests fail with precision?

Fixed - using precision wasn't working if a NULL value was present in an attribute

[nyalldawson]

@nirvn

Nice speed gain: 20.89sec (python) vs 15.36sec (native c++) for a large-ish polygon dataset resulting in 51,000 points.

I take your paltry 25% speed gain and raise you a 98% speed gain: #5621 ;)

[nirvn]

@nyalldawson , I am once again humbled 😉