Add `__eq__` method to class `FDerivativeOperator`

[Samuel Lelièvre]

Define a function and its derivative:

sage: U = function('U')
sage: U(x).diff(x)
diff(U(x), x)
sage: U(x).diff(x).operator()
D[0](U)

Before this ticket:

sage: U(x).diff(x).operator() == U(x).diff(x).operator()
False

After this ticket:

sage: U(x).diff(x).operator() == U(x).diff(x).operator()
True

[Samuel Lelièvre]

Initial discussion:

• ​Ask Sage question 54555: Substitute U' to D[0]U

[Matthias Köppe]

Setting new milestone based on a cursory review of ticket status, priority, and last modification date.