external_force_fcn

abstract interface
private subroutine external_force_fcn(self, particle, pairs, acc, n)

Type	Intent		Name
class(external_force_type),	intent(inout)	::	self
type(particle_type),	intent(in)	::	particle	particle 区域
type(nnps_pairs_type),	intent(in)	::	pairs	Pairs 粒子对
real(kind=rk),	intent(inout)	::	acc(:,:)	Acceleration 加速度
integer,	intent(in)	::	n	实粒子数, nreal

Solve the external force
外力求解，涉及重力和边界力