Implement direct conversion functions

[tmilnthorp]

Implementing direct conversion functions would help reduce error. Currently, to go from ounces to pounds, we go: ounces -> kilograms -> pounds

This shows an example of going directly from ounces -> pounds.

Long term, we could move all FromUnitToBaseFunc/FromBaseToUnitFunc into this conversions array.

Just a quick example to show what's on my mind.