Computes the norm of subvector of a vector defined by a starting point and a stride.
nrm - the norm
If x is the array representing the vector x then this computes the norm of the array (x[start],x[start+stride],x[start+2*stride], ….)
This is useful for computing, say the norm of the pressure variable when the pressure is stored (interlaced) with other variables, say density etc.
This will only work if the desire subvector is a stride subvector