use factory methods for unit conversions

[gidden]

every parameter entry of the form [parameter, value] has a corresponding unit associated with it. accordingly, a family of factory converter classes would be appropriate.

e.g., if power is given in MW, GW, kW, W, etc., I would like to be able to use the following

power = PowerConverter(units, value).get()

this will allow us to house the specific cyclus definition of each parameter in a separate class, providing for easy ability to update when/if cyclus defaults change.