Added operator overloads for PLD composition.

[lumip]

Suggestion: Simple composition of pld1, pld2 can now be expressed using pld1 + pld2, k-fold self-compositition as k * pld1. This allows for easy-to-read construction of complex PLD compositions, e.g., pld = k12 * (k1 * pld1 + pld2) + k3 * pld3 instead of pld = pld1.self_compose(k1).compose(pld2).self_compose(k12).compose(pld3.self_compose(k3)).