Raytracing

- Hundreds of new lenses thanks to Alex Tchung
- Small modifications to allow integration in Tkinter (coming soon!)

What's Changed
* Fixed version check by actually writing a test for it. by dccote in https://github.com/DCC-Lab/RayTracing/pull/453
* Better warnings by dccote in https://github.com/DCC-Lab/RayTracing/pull/458
* Gaussian rays by dccote in https://github.com/DCC-Lab/RayTracing/pull/459
* Fixed display bug by dccote in https://github.com/DCC-Lab/RayTracing/pull/457
* Minor fix to keep python 3.6 support by JLBegin in https://github.com/DCC-Lab/RayTracing/pull/463


**Full Changelog**: https://github.com/DCC-Lab/RayTracing/compare/1.3.9...1.3.10

What's Changed
* Small convenenience functions: ToConjugate(), ToFocus(matrix), Lens().focusToFocus()
* Fix various display properties by JLBegin in https://github.com/DCC-Lab/RayTracing/pull/437
* Fix bug in GaussianBeam where wavelength wasn't converted to nm for printing by mroddewig in https://github.com/DCC-Lab/RayTracing/pull/443
* Better compound element figure by dccote in https://github.com/DCC-Lab/RayTracing/pull/445
* Version checking: We timestamp even when we fail: no need to keep checking by dccote in https://github.com/DCC-Lab/RayTracing/pull/448
* Preferences, better version check and convenience functions by dccote in https://github.com/DCC-Lab/RayTracing/pull/450

New Contributors
* mroddewig made their first contribution in https://github.com/DCC-Lab/RayTracing/pull/443

**Full Changelog**: https://github.com/DCC-Lab/RayTracing/compare/1.3.7...1.3.9

Simple addition: aperture now has an optional parameter NA which can act as a limiting NA for the rays. Therefore, you can for instance use an aperture with a finite diameter and finite NA to simulate an optical fibre.

Thanks for the feedback, a few improvements and a few bug fixes:
1. Final small changes to examples. Use `python -m raytracing` to see all examples or `python -m raytracing -l` to list all examples.
2. Bug fix for the display of some lenses with very small radii of curvature.
3. There is an expert mode to limit the warning. Define the environment variable RAYTRACING_EXPERT=1 to minimize the warnings.
4. The package will quickly check (if possible) on pypi to see if there is a newer version of `raytracing` and will let you know how to upgrade. It will only check every once a day.

* `imageSize` was modified to address confusion: in optical design, the object is always the field of view, and the image size is naturally the magnifation x FOV. However, although in real systems the FOV is always finite, it is not the case in small systems with infinite diameters, often used to get started with calculations. This led to confusion, because imageSize was infinite with no Field Stop is present (at least two finite diameter elements). So now, imageSize will offer to use the finite `objectHeight` of `ImagingPath` for the calculation if the FOV is infinite. It is a new parameter `imageSize(useObject=False)` that defaults to the same behaviour as before.
* Documentation has been improved significantly: better figures (up to date), syntax coloring on graphs
* Example code is now distributed with the module and is accessible with `import raytracing.examples`
* And all the goodies of 1.3.4