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

[nielstron]

solved by #225