Dashed bonds for bond order < 1 (revised)

[dxdc]

Builds on https://github.com/3dmol/3Dmol.js/pull/641

• Previously the dash calculation was only integrated into drawDashedStickImposter. This was inflexible, as there was no native support for dashes in GLDraw.drawCylinder.

• Updated dash calculation algorithm. Seems to result in improved behavior.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (2e1c453) 83.17% compared to head (84c512a) 83.20%.

Files	Patch %	Lines
src/GLModel.ts	94.73%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #740 +/- ## ========================================== + Coverage 83.17% 83.20% +0.03% ========================================== Files 141 141 Lines 11484 11488 +4 Branches 2126 2125 -1 ========================================== + Hits 9552 9559 +7 + Misses 1602 1599 -3 Partials 330 330 ```

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[dxdc]

@dkoes one comment here:

gapLength = (cylinderLength - totalDashLength) / totalSegments;

This is actually not the correct way to calculate it. Probably it should be more exactly something like:

gapLength = totalSegments > 1 ? (cylinderLength - totalDashLength) / (totalSegments - 1) : 0;

However, that yields some strange behavior as far as the cylinder coloration - because it looks as if the bonds are created in equal halves - and you can get some half bonds of differing colors.

I'm not an expert on the downstream steps here, and I'm not entirely sure why having a slightly smaller gapLength fixes that issue. I imagine there is something more robust.

[dkoes]

I'm not sure what the issue is with coloration - the test cases look like an improvement to me, so I'm going to go ahead and merge.