Defensive programming

[AleMorales]

Make sure that inputs passed to the public API are always valid. We should not repeat assertions in internal functions but that probably means wrapping inputs into dedicated data types such that assertions are only made once when constructing the data type. Some things may be hard to detect automatically like:

• Using different lengths for power in materials and light sources (maybe a check inside the material?)
• Incorrectly using the grid cloner (rely on templates provided as how-to guides)
• Using the wrong units (e.g., are angles in radians or hexadecimal degrees). Here we could go for consistency: public API always in degrees, and internally always in radians.