[BUG] - syntaxWarning in constitutive models

[bknight1]

Describe the bug

• What is the bug?

../underworld3/src/underworld3/constitutive_models.py:634: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if inner_self.yield_stress_min.value is not 0:

• What version code?
  latest dev branch, python-3.10.14
• What system are you running the code on?
  M2 - arm
• Steps to reproduce?
  Fresh install of updated dev branch

Screenshots

[bknight1]

Few 'is not' statements in the file, not sure if we want to update them all

[julesghub]

Yeah I say we update them.
!= is a test about equality of value.
is not is a test about the identify of the python object.

[bknight1]

Fixed in #222