Merge pull request #1685 from lengxupa/master

         fix-ID = ID of :doc:`fix rigid/small <fix_rigid>` command

       *shake* value = fix-ID

         fix-ID = ID of :doc:`fix shake <fix_shake>` command

       *orient* values = rx ry rz

         rx,ry,rz = vector to randomly rotate an inserted molecule around

       *units* value = *lattice* or *box*

         lattice = the geometry is defined in lattice units

         box = the geometry is defined in simulation box units

all incident particles strike the surface as if they are in an

incident beam of particles at a prescribed angle.

The *orient* keyword is only used when molecules are deposited.  By

default, each molecule is inserted at a random orientation.  If this

keyword is specified, then (rx,ry,rz) is used as an orientation

vector, and each inserted molecule is rotated around that vector with

a random value from zero to 2*PI.  For a 2d simulation, rx = ry = 0.0

is required, since rotations can only be performed around the z axis.

The *id* keyword determines how atom IDs and molecule IDs are assigned

to newly deposited particles.  Molecule IDs are only assigned if

molecules are being inserted.  For the *max* setting, the atom and