Added resize parameters for dataset image reader, and the ability to loa...

...d data from alternate directories

cc @eickenberg 

Added the ability to specify arbitrary directories for the load_sample_images and load_sample_image functions to enable loading "other data" easily. 
so here the `endswith` kind of glosses over that. You can give it full path, only basename, half of the full path, and all of it will work. Worse, you can give it e.g. `'e.jpg'` and it will select the first image complying with the glob `'*e.jpg'`. Imagine you have composite words ... :)