Physical constants?

[billyrieger]

Thanks for this great library! I think it'd be a cool feature to have physical constants directly integrated with uom. Existing crates such as natural_constants and physical_constants don't use units at all and instead just have the constants as floating-point numbers.

Some considerations:

• Physical constants could be considered outside the scope of uom and might be better suited to being defined in their own crate (uom-constants perhaps).
• If constants are included in uom, they should probably be gated behind a feature flag.
• The values of the constants could be taken from NIST, but deciding which constants to include and which to ignore is also an issue.
• I'm not entirely sure how constants would interact with the different underlying storage types. Some constants are exact integers (e.g. speed of light in vacuum) whereas most are floating-point and have some uncertainty.

I'd be happy to begin work on this feature if you think it's suitable for uom and the above considerations are resolved.

[iliekturtles]

Duplicate of #32. Closing this one and I will reply there. Definitely something I would like to see.