Clean up bead naming
Add ability to get absolute z position of optical elements
Add PositionSpan source
Update test drawing
working
Update Naming for Sources and Detectors
Add Concrete Ray source (and naming updates)

* Added unicode function to ray object
* Change order of a and z arguments in Ray initializer (since z is less
  likely to change)
* Add ConcreteSource and SingleRaySource
* Add new test to Bead
update test for single known ray with a bead
merge