Curle's analogy

I believe there is a mistake in the implemented acoustic analogy in OF-1706. The coefficient on the RHS should be 1/(4pi) instead of 4pi.

Probably change: pDash = 4*mathematical::pi/c0_ * (d/magSqr(d) & dfdt); to pDash = 1/4/mathematical::pi/c0_ * (d/magSqr(d) & dfdt);