CODATA support

[ggggggggg]

The values of physical constants should come from CODATA and include uncertainty. For now I don't see a clear path towards including covariance between constants in a way that propagates, so I think we just have to have a disclaimer saying the covariance is treated as 0. Here is a proposed path

• Create a PhysicalConstant type that acts as an Uncertain when interacting with Uncertain or Quantity{Uncertain}, and otherwise acts as a Quantity.
• Create a mapping between CODATA names and Physical names, and a loop that uses that mapping to create PhysicalConstant variables in the Physical namespace.