* Add per-pixel mean square loss

* Add documentation of loss_mean_squared_per_pixel_

* Add test case for per-pixel mean square loss: a simple autoencoder

* Review fix: reorder params of function tensor_index, so that the order corresponds to the convention used in the rest of the dlib code base

* Review fix: add breaks as intended, and change the rest of the test accordingly

* Again a case where the tests already work locally for me, but not on AppVeyor/Travis - this commit is a blindfolded attempt to fix the problem
(and it also fixes a compiler warning)