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. 
What are you expecting here as `image_name`? The basename or the full path or both? I would prefer using `os.path.basename` instead of endswith. This may make the check easier, too.