Moved code to compat subpackage and ajusted imports
Added TransformDict from 3.4
Implemented function and cache to get equivalent units.

Usage:

>>> ureg = pint.UnitRegistry()
>>> ureg.get_equivalent_units('meter')

but also:

>>> ureg.get_equivalent_units('[length]')

Todo:
- order output according to some logical criteria
- speed up process to build cache
- enable contexts for this operation