Primary system acceleration
@elena so, as described in #258 (closed), we need a function that computes \int_{v\in disk}\frac{mass(v)\vec{v}}{{|v|}^3}d{v}
@elena so, as described in #258 (closed), we need a function that computes \int_{v\in disk}\frac{mass(v)\vec{v}}{{|v|}^3}d{v}