title: Mean of non-zero elements in 3d array
body: <p>I have this i x j x k 3d matrix (it's a movie). Without loops, I'm trying to take the mean of the non-zero positive elements in each ixj array and put these values into a 1x1xk matrix.</p>
  
  <p>I've been searching for quite a while now, and although there's plenty of solutions to accomplish this for a 2d matrix, I can't for the life of me find a way to do it for a 3d matrix without using a loop.</p>