Pvtrace

* Resigned much of the code base!
* Added wavelength selective coating using `SurfaceDelegate` approach.
* Added scattering material
* Renamed Lumophore to Luminophore
* Added phase functions which can be used by scattering and luminophore to control scattering and reemission directions
* This version will break scripts written for earlier releases.
* More documentation.

Bug fix release

* Fixes issue 23

Refactoring of photon tracer algorithm and other simplifications.

Added cylinder geometry.

* Requires Python 3 (only tested with Python 3.7). The utility 2to3 has been run on the code base.

* Conda `environment.yml` added for creating runtime environment.

* The visualises has been updated to work with VPython 7. This integrates nicely with Jupyter Notebooks.