System of Equation not supporting built-in Equations

[GiovanniCanali]

Describe the bug
The class SystemEquation does not allow to create a system with built-in Equation. This behaviour is odd and counter-intuitive.

To Reproduce
Consider a Navier-Stokes problem with no-slip boundary conditions on some walls. One would like to write the following:

"wall": Condition(
    input=wall_pts,
    equation=SystemEquation(
        [
            FixedValue(value=0.0, components=["u"]),
            FixedValue(value=0.0, components=["v"]),
            FixedGradient(value=0.0, components=["p"], d=["y"]),
        ]
    ),
),

Expected behavior
This should work without any problem.

Output

ValueError: equation must be a callable function.Expected a callable function, ...