ENH: Added new Curle function object

Calculates the acoustic pressure based on Curle's analogy.

Curle's analogy is implemented as:

    \f[
        p' = 4 \frac{\pi}{c_0}\frac{\vec d}{|\vec d|^2}\frac{d(F)}{d(t)}
    \f]

where
    p'          | Curle's acoustic pressure [Pa] or [Pa (m3/rho)]
    c_0         | Reference speed of sound [m/s]
    \vec d      | Distance vector to observer locations [m]
    F           | Force [N] or [N (m3/rho)]