Initial Cut of Unit-Aware Value System ( Length )

[joeyame]

Part of making a good, easy to use sim framework mandates a physical value-tracking system that ensures unit-safe operations.

Such a system should have 3 properties (for now just assume length):

1. Lengths can be added, subtracted from each other.
2. Lengths can be scaled by a given unitless factor.
3. Trying to multiply lengths together causes an error ( no Area types exist yet )
4. We can pull any length unit out of a Length object and it will automatically convert as needed (1m.get(feet) == 3.28084)

[joeyame]

Investigate:

1. https://crates.io/crates/quantities
2. https://crates.io/crates/fts_units
3. https://crates.io/crates/dimensioned

These libraries either provide their own units, or give devs the ability to define their own.