Approve change to order of unit definitions

[timbrisc]

Related to issue 289, the order of definitions should be applied in ucum-essence.xml as identified by NLM's LHC group.

Changes in the order of the unit definitions

UCUM definitions build on the base units. For example, the code N for newton is built from kg.m/s2, which means that the units for g (gram - with the prefix k indicating a kilogram), m (meter), and s (second) must all be defined before the newton unit can be defined.

The ucum-essence.xml file is therefore processed sequentially, so that units can be built on previous definitions. Every once in a while the unit definitions in the ucum-essence.xml don't follow this convention, with the definition of one unit being based on another unit whose definition has not yet been read. See the list below for these instances. Comments have been inserted in the ucum-essence.xml file to document the moves.

The order of definitions for gon and deg were switched. gon was defined immediately before deg but gon is built on deg (deg.(0.9)).
The definition of eV was moved from a point preceding [e] to a point following [e]. eV is defined by [e] ([e].V).
The definition of [lbf_av] was moved from a point preceding [lb_av] to a point following [lb_av]. [lbf_av] is defined by [lb_av] ([lb_av]/16).