More robust Sankey

[alexcjohnson]

Make Sankey more accepting of messy data (fixing #2480)

• Numeric strings are OK
• Zero (and negative) link values get ignored completely. Note: this is not what I proposed in Sankey nodes overlap by default #2480 (comment) - thinking a bit more about it, links with no value should really be not links at all, and nodes with no size shouldn't be nodes at all. If you really want a node & link to show up with zero value, create one with an explicitly tiny value.
• Unused nodes and malformed links are ignored as well. Since there are now various places this can happen and in other contexts we simply ignore partially-invalid data points, I also removed the logged error on missing nodes - which happened during supplyDefaults, so was called frequently, and had a fairly slow implementation so could be a drag on large Sankey traces.

cc @etpinard

[alexcjohnson]

I initially made isIndex into a Lib function thinking I could reuse it here... but in this context it's in principle important that an index is actually a number, to distinguish an array index from an object property that happens to be a number. That said I don't think it's possible to use a number as an object property in nestedProperty...

[etpinard]

Oh we could use this for validating selectedpoints items as done here:

plotly.js/src/lib/index.js

Lines 497 to 499 in a132b85

	function isPtIndexValid(v) {
	return isNumeric(v) && v >= 0 && v % 1 === 0;
	}

[etpinard]

... by this I mean Lib.isIndex

[alexcjohnson]

Good call -> cdcd620

We probably either have or should have this in other places too... mesh3d is the other obvious place, but it has bigger issues -> #2630

[alexcjohnson]

For the most part it didn't cause problems to have extra unused nodes in the list, but sometimes the layout algorithm would reserve extra space for these nodes (leaving extra margins or making the whole diagram S-shaped, for example) so it's better to remove them. The sankey_messy mock includes a whole bunch of extra nodes both before (to test this link reassignment) and after the real ones to test this.

[etpinard]

Would be nice to reuse:

plotly.js/src/lib/matrix.js

Lines 13 to 17 in a132b85

	exports.init2dArray = function(rowLength, colLength) {
	var array = new Array(rowLength);
	for(var i = 0; i < rowLength; i++) array[i] = new Array(colLength);
	return array;
	};

[etpinard]

nicely done 👌

[etpinard]

Great PR 💃

[etpinard]

... my comments are non-blocking.