(addressing slabbe's questions from #33092 comment:23)

Some questions arise with respect to is_present(),
is_functional() and require():

• why does Feature.require() not check
  that is_functional() is True?
• should is_present() always return False
  when is_functional returns False (while
  the method names suggest is_present could
  be True while is_functional is False)?

Choices made in this ticket:

• only is_present() should be called from outside
• it always checks that is_functional() returns True
• the require() method only calls is_present()

A further step, not done in this ticket, would be,
as suggested in comment:14, to rename is_functional
to _is_functional, since only is_present is supposed
to be used from outside.

CC: @seblabbe @orlitzky @kwankyu @saraedum

Component: refactoring

Author: Matthias Koeppe

Branch: f9232e8

Reviewer: Sébastien Labbé

Issue created by migration from https://trac.sagemath.org/ticket/33114