WIP: Allow use of symbolic function in equation and add test

[schymans]

In the current version, it is not possible to use symbolic functions in Equation(), e.g.:

from sympy import Derivative, Eq, Function, symbols
from essm.variables import Variable
from essm.equations import Equation
from essm.variables.units import meter, second

class v(Variable):
    """Velocity"""
    unit = meter / second

class d(Variable):
    """Distance"""
    unit = meter
    
class t(Variable):
    """Time"""
    unit = second

dt = Function(d)(t)    
class eq_v(Equation):
    """Average velocity as a function of time"""
    expr = Eq(v, Derivative(dt, t))

ValueError: Arguments in function are not dimensionless, but have dimensions of {Dimension(time)}

This PR disables the requirement that arguments in functions must be dimensionless.

[schymans]

Now the error is different:

dt = Function(d)(t)    
class eq_v(Equation):
    """Average velocity as a function of time"""
    expr = Eq(v, Derivative(dt, t))

ValueError: Dimension of "v" is Dimension(length/time), but it should be the same as Derivative(d(t), t), i.e. Dimension(1/time)
This is because the symbolic function dt did not inherit the units of d.