Supply both slope and aspect to shader in radians

The **czm_materialInput.slope** parameter should be in radians. To fix this, simply return the arc-cosine of the currently computed slope. Also, there should be a **czm_materialInput.aspect** parameter supplied to the shader that is also in radians. Zero radians should mean _East_, pi/2 radians should mean _North_, pi radians should mean _West_, and 3pi/2 radians should mean _South_.

I have forked the cesium repo and am trying to get a pull-request together for this new feature. The math is not hard given the needed inputs for the calculation. The challenge is getting those inputs.

Can anyone concur that this is a good idea? I really need aspect along with slope in a Cesium-based project I am currently working on.

Start by locating where czm_materialInput.slope is computed and where shader material inputs are supplied; the issue names no file or test. Compare the existing slope inputs with the requested radians and define the aspect calculation using the stated East/North/West/South convention. Done means shaders receive both slope and aspect in the requested units.

Written by the indexing model from the issue text.