function getU(matrix, m, n, l) {
  // Although [1, 2] split U into three cases for m == 0, m < 0, m > 0
  // the actual values are the same for all three cases.
  return getP(matrix, 0, m, n, l);
}
The functions U, V, and W should only be called if the correspondingly
named coefficient u, v, w from the function ComputeUVWCoeff() is non-zero.
When the coefficient is 0, these would attempt to access matrix elements that
are out of bounds. The vector of rotations, |r|, must have the |l - 1|
previously completed band rotations. These functions are valid for |l >= 2|.
@param {Number[]} matrix - N matrices of gainNodes, each with (2n+1) x (2n+1)
elements, where n=1,2,...,N.
@param {Number} m
@param {Number} n
@param {Number} l
@return {Number}