Refacto `command.py` to have a proper `class Command`

[benjvmin93]

Is your feature request related to a problem? Please describe.
Right now, there is a class _KindChecker that ensures the existence of a kind attribute for the commands. This class is not suited to implement class methods for the commands.

Describe the feature you'd like
It may be better to have a proper class Command that could inherit from _KindChecker in order to keep ensuring the kind attribute are setted. This class will be useful to put methods related to converting commands into latex, ascii or unicode string representations.

Additional context
This issue is related to PR#245 and follows the discussion on turning the functions for drawing commands into class methods.

[thierry-martinez]

The discussion in #245 is here: #245 (comment)