fixed bug in MnistImageFile::numEntries()

Change-Id: Ib3d45d8555cfab3f511bbfe55ecf1a5e1de66b0a
testrbm8.8.2

Change-Id: Ie898ee955c9657b96b78a74233404d1b9010045d
testrbm16.16.2

Change-Id: I673f1ec63ca67ced2469c4d951edc5f07e86e716
look at more images on more pixels on hardware

Change-Id: I65554f938096116b3f1929e52ee3c47ad0e9a074