Handle overflows in conversion factors

[iliekturtles]

Handle overflows in conversion factors so that factors that exceed the underlying storage type's min/max values do not cause errors. Currently tests are failing because conversion factors overflow the underlying storage type.

• Don't implement Conversion for types that can't represent the conversion factor? e.g. don't implement Conversion for kilometer when the underlying storage type is u8.
• Implement Conversion but have tests skip factors that can't accurately be represented?