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 
    ...
    } 


(4) conversion test is managed via rmse improvement (.e.g. threshold = 0.1 means bail out if rmse didn't improve by more than at least 10% compared to previous iteration's rmse). 

0 or negative threshold means "just run all iterations, don't bother with rmse computations". 

(5) added DRM's norm summary.