Use module function and do not instantiate table unnecessarily

[kmcphillips]

@thegedge @benwah

Closes #38

Problem

Unnecessary to create an instance of the conversion table class to call a single function and throw it away.

Solution

Use a module function. Trade off is that we have to pass units down all the way through in a bunch of places.

In the 2.0 milestone, but doesn't change the interface at all for the 1.x version.

[kmcphillips]

If you feel strongly I'm ok with a 2.0 branch to save releases here. Doesn't really matter.

[kmcphillips]

Thanks