Medimages4tests

Detects whether the desired dummy nifti file is compressed or not based on the provided suffix (if any). If no suffix is provided, then the appropriate suffix is determined by the compressed variable

This release adds real MRI images to the suite by downloading them from [openneuro](https://openneuro.org/).

To make room for these "real" images in the package hierarchy, the existing methods that generate dummy images have been moved inside a `dummy` sub-package, and the standard method used to retrieve the images have been renamed from `sample_image` to `get_image`

Adds support for NIfTI sample images