use gt/lt instead of is_same_dimensionality

* add dimensionless to registry
* add eq to unit
* fix unit times for dimensionless
* move Constant properties to top of file
use unit class instead of string for units property

* remove unit dependent property and get/set methods
* refactor as needed
* default unit class value is 1 for dimensionless
* use value = 1 for all base (eg: SI) units
* fix display of unit class for base units
* refactor quantity calls to use unit class not string