Interpolate being both a BaseForm and Expression causes issues

I have encountered the following 2 cases where `Interpolate` being both an `Expr` (i.e. primal) and `BaseForm` (i.e. dual) object causes things to break down.

### Case 1

```py
u = Coefficient(V1)
Iu = Interpolate(u, V2)
uhat = TrialFunction(V1)

FormSum((Iu, 1), (Iu, 1)).arguments() # works fine
(Iu + Iu).arguments()
```
gives
```
> (Iu + Iu).arguments()
^^^^^^^^^^^^^^^^^^^
E AttributeError: 'Sum' object has no attribute 'arguments'
```

### Case 2

```py
u = Coefficient(V1)
Iu = Interpolate(u, V2)
uhat = TrialFunction(V1)

independent = Coefficient(V1)
dIu = derivative(Iu, independent, uhat)

# Sum(ZeroBaseForm, ZeroBaseForm)
expand_derivatives(dIu + dIu)
```
gives
```
def __new__(cls, a, b):
"""Create a new Sum."""
from ufl import BaseForm, FormSum

# Base forms (that aren't also expressions like Interpolate) should
# be cast to FormSums instead.
# if any(isinstance(x, BaseForm) and not isinstance(x, Expr) for x in [a, b]):
# return FormSum((a, 1),(b, 1))

# Make sure everything is an Expr
a = as_ufl(a)
b = as_ufl(b)

# Assert consistent tensor properties
> sh = a.ufl_shape
^^^^^^^^^^^
E AttributeError: 'ZeroBaseForm' object has no attribute 'ufl_shape'

ufl/algebra.py:49: AttributeError
```

These tests are oddly specific but the errors arose inside a complicated piece of adjoint logic where we were taking derivatives of interpolates and summing them together. `Sum(BaseFormOperatorDerivative, BaseFormOperatorDerivative)` is allowed but `Sum(ZeroBaseForm, ZeroBaseForm)` (which naturally arises during `expand_derivatives`) is not.

To me it seems likely that the solution is something complicated to do with the type system. For now we will just avoid the case that triggers this.