Note: the word 'plain' was chosen instead of 'base' as BaseUnit or BaseQuantity
as the latter might be interpreted physically and not a part of the class hierarchy
as intented.

add a example for a custom unit formatter

* ignore magnitude formats for units

* convert non-truthy to empty string

* match longer flags first

i.e. match `cfunits` before `cf`. There might be a better way to do
this, though.

* add a test to check for the deprecation

* consistently split between mspec and uspec

which allows emitting a DeprecationWarning

* correct the explanation of where the default format is coming from

* use `reverse=True` instead of sorting with `-len(x)`

Co-authored-by: Jules Chéron <43635101+jules-ch@users.noreply.github.com>

* make sure the warning is raised correctly