Refactor image loading code to handle mode file types in a more standardized way

[ieee8023]

Add a utility function that takes in a path (jpg, png, dcm, etc) and returns a standardized tensor. Most of this code is in the get function on the dataloaders and it should be factored out.

Possibly in a new namespace like xrv.utils.load_img

This will make it very easy to include dcm support and also make tools which load and process images.