Mahout 1566 Regular ALS factorizer with convergence test

Few things happened here : 

(1) default CacheHint for implicit optimizer checkpoint is changed to NONE (used to be: MEMORY-ONLY). 
(2) bug fixes in the AB' physical operator found. 
(3) added implicit drm -> inCore conversion which allows ALS iteration in its simplest form to look simply like 

    while (!stop && i < maxIterations)  {

      drmV = drmAt %*% drmU %*% solve(drmU.t %*% drmU)
      drmU = drmA %*% drmV %*% solve(drmV.t %*% drmV)

     ...
     i += 1 
    ...
    }