Medimages4tests

Added in raw data file generation for Siemens PET Biograph

What's Changed
* Bug fixes by tclose in https://github.com/Australian-Imaging-Service/medimages4tests/pull/3


**Full Changelog**: https://github.com/Australian-Imaging-Service/medimages4tests/compare/v0.3.1...v0.3.2

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