Consider adding logical_xor to mathematical functions

[jlalbers]

Currently, an XOR symbol can be created by composing different logical function symbols, i.e.:

sym_1 = model.binary(...)
sym_2 = model.binary(...)
xor = logical_and(
    logical_not(logical_and(sym_1, sym_2)),
    logical_or(sym_1, sym_2)
)
...

However, I imagine it would be more efficient and expressive to have a separate XOR symbol.

[arcondello]

Do you have a specific use case in mind? I agree it could be there for symmetry, but I'd like to add new nodes lazily rather than eagerly.