Units are not consistent with Pyomo DAE discretization

[dallan-keylogic]

Summary

Units aren't involved in pyomo.dae discretization transformations, which leads to inconsistent unit errors.

Steps to reproduce the issue

import pyomo.environ as pyo
from pyomo.dae import ContinuousSet, DerivativeVar
from pyomo.util.check_units import assert_units_consistent

m = pyo.ConcreteModel()
m.time = ContinuousSet(initialize=[0, 1, 2, 3, 4])

m.x = pyo.Var(m.time, units=pyo.units.mol, initialize=0)

m.dxdt = DerivativeVar(m.x, wrt=m.time, initialize=0, units=pyo.units.mol/pyo.units.s)

pyo.TransformationFactory("dae.finite_difference").apply_to(m, nfe=len(m.time)-1, wrt=m.time, scheme="BACKWARD")

assert_units_consistent(m)

Error Message

Exception has occurred: InconsistentUnitsError
Error in units found in expression: dxdt[1] - (x[1] - x[0]): mole / second not compatible with mole.
  File "C:\Users\[REDACTED]\Repos\pyomo\pyomo\core\base\units_container.py", line 482, in _get_unit_for_equivalent_children
    raise InconsistentUnitsError(
  File "C:\Users\[REDACTED]\Repos\pyomo\pyomo\core\base\units_container.py", line 929, in exitNode
    return node_func(self, node, data)
  File "C:\Users\[REDACTED]\Repos\pyomo\pyomo\core\expr\visitor.py", line 481, in _process_node_bx
    return self.exitNode(node, data)
  File "C:\Users\[REDACTED]\Repos\pyomo\pyomo\core\expr\visitor.py", line 268, in walk_expression
    result = self._process_node(root, RECURSION_LIMIT)
  File "C:\Users\[REDACTED]\Repos\pyomo\pyomo\core\base\units_container.py", line 1206, in _get_pint_units
    return self._pintUnitExtractionVisitor.walk_expression(expr=expr)
  File "C:\Users\[REDACTED]\Repos\pyomo\pyomo\util\check_units.py", line 165, in _assert_units_consistent_expression
    pint_unit = units._get_pint_units(expr)
  File "C:\Users\[REDACTED]\Repos\pyomo\pyomo\util\check_units.py", line 249, in assert_units_consistent
    _assert_units_consistent_expression(obj)
  File "C:\Users\[REDACTED]\Repos\pyomo\pyomo\util\check_units.py", line 121, in _assert_units_consistent_constraint_data
    assert_units_consistent(*args)
  File "C:\Users\[REDACTED]\Repos\pyomo\pyomo\util\check_units.py", line 270, in assert_units_consistent
    handler(cdata)
  File "C:\Users\[REDACTED]\Repos\pyomo\pyomo\util\check_units.py", line 193, in _assert_units_consistent_block
    assert_units_consistent(component)
  File "C:\Users\[REDACTED]\Repos\pyomo\pyomo\util\check_units.py", line 275, in assert_units_consistent
    handler(obj)
  File "C:\Users\[REDACTED]\Desktop\discretization_units.py", line 14, in <module>
    assert_units_consistent(m)
pyomo.core.base.units_container.InconsistentUnitsError: Error in units found in expression: dxdt[1] - (x[1] - x[0]): mole / second not compatible with mole.

[blnicho]

This is a duplicate of #1790