Generate additively separable representations of expressions

[emma58]

Summary

The _generate_additively_separable_repn function used in gdp.partition_disjuncts handles only the easiest case where the expression in question is a SumExpression. There are a lot of other cases that we could handle:

• {not_potentially_variable} * (m.x + m.y)
• (m.x + m.y) / {not_potentially_variable}
• recursive forms of the above

Rationale

If automatically partitioning is useful in between steps of if others want to rely on this functionality, then it should be more general.