Unit registries

[rgm]

Every one of our projects seems to require a slightly different set of units. A central concept in Pint is user-definable registries. Let's bring that in, and make the make-quantity fn take a unit registry as its first argument so that the validation, etc. can vary per project.

Proposed API:

(require '[opengb.dram.core :as dram])

(def unit-reg (-> (dram/make-unit-registry)
                  (dram/define-unit "air-change = [airflow] =  ac")
                  (dram/define-unit "[airtightness] = [airflow] / [time]")))

(def Q_ (partial dram/make-quantity unit-registry))