Enable adding custom units and entities to quantulum

[nielstron]

Is your feature request related to a problem? Please describe. There may be very specific units to some contexts which are not yet (or should not be) supported by the default quantulum distribution.

Describe the solution you'd like A simple way to add custom units and entities to be parsed (and consequently one to remove them as well) (i.e. parser.add_unit(...), parser.remove_unit(...))

Describe alternatives you've considered None

Additional context #135