Add aggregated values to CLI logger

[tancik]

It would be nice to have some aggregated stats in the CLI logger. Also have it display the last Test PSNR value. Given that there is no scrolling, it would be nice to somehow get an idea of test psnr without having to wait for the next test.

Bonus: If we could report the last test PSNR with a little arrow and delta that tells you if it is increasing or decreasing.

Step (% Done)       Data Load (time)     Train Iter (time)    Rays Per Sec         Test PSNR    
--------------------------------------------------------------------------------------------------------
1730 (0.17%)        1.754 ms             23.879 ms            360671.5761                               
1740 (0.17%)        1.858 ms             23.327 ms            361620.5520                               
1750 (0.18%)        1.857 ms             23.335 ms            359403.9703                               
1760 (0.18%)        1.852 ms             23.861 ms            246720.2215                               
1770 (0.18%)        1.950 ms             23.339 ms            358698.5945                               
1780 (0.18%)        1.851 ms             23.374 ms            357221.8240                               
1790 (0.18%)        1.853 ms             23.356 ms            359550.6458                               
1800 (0.18%)        1.854 ms             23.314 ms            359592.0375                               
1810 (0.18%)        1.752 ms             23.865 ms            357932.5837                               
1820 (0.18%)        1.850 ms             23.349 ms            361498.8045   
--------------------------------------------------------------------------------------------------------
ETA                 Avg Data Load         Avg Train           Avg rays/sec        Last Test PSNR
 6 h 30 m           1.813 ms             23.700 ms             350000               20.45