Gathers a single component from a multi-component vector into another vector.
v - the vector
start - starting point of the subvector (defined by a stride)
addv - one of ADD_VALUES,INSERT_VALUES,MAX_VALUES
s - the location where the subvector is stored
One must call VecSetBlockSize() before this routine to set the stride information, or use a vector created from a multicomponent DMDA.
If x is the array representing the vector x then this gathers the array (x[start],x[start+stride],x[start+2*stride], ….)
The parallel layout of the vector and the subvector must be the same; i.e., nlocal of v = stride*(nlocal of s)
Not optimized; could be easily