Opt-out Z interpolation

[bjornharrtell]

ref #714

Wanted to see what effect it would have to turn interpolation off by default and it causes a number of expected test failures (OverlayNGZTest, RobustLineIntersectorZTest and GeometryFixerTest).

If we agree that #714 is indeed a regression then I would have no better current idea than doing something like this, unfortunately.

[dr-jts]

Using a global option set via System property seems like the simplest way to allow a choice of Z-interpolation behaviour (although it's the least flexible).

The unit test issues can be handled by either making Z-interpolation the default, or by changing the option setting in the unit test. The latter approach is already used in the GeometryOverlayTest.

[bjornharrtell]

@dr-jts I've opted in the interpolation in the test cases where it is the assumption.

[bjornharrtell]

I've now reworked this so that the option to Z-interpolate is standalone and affects both RobustLineIntersector and ElevationModel.

[jodygarnett]

This seems a lot of boilerplate to hold a boolean value decision, can this class take on more responsibility and swap between interpolation approaches as a stratagy object?

[bjornharrtell]

I agree it feel like boilerplate. I have difficulties properly understanding/interpreting what you mean with "take on more responsibility and swap between interpolation approaches as a stratagy object". In either case it's intentionally done this way because it is modeled after the other existing system property option jts.overlay and broken out into a self contained class because it is affecting two very separate things (RobustLineIntersector and ElevationModel), and I wanted to do it in a way that is as non intrusive as possible on the existing source.

I suppose system wide settings could be done with smarter and nicer desig but I don't think this is the place to do such general improvement and restructuring.

[bjornharrtell]

Since Z interpolation has now been enabled for so long time it's no longer sensible to regard it as a regression. So I've changed it to be default on in this PR. This means this PR now do not change any behaviour and only allows the interpolation to be opt out.

[bjornharrtell]

@jodygarnett and @dr-jts please consider accepting this.