Unification of pl-med2img, pfdo_med2image, pl-pfdo_med2img

[`pl-med2img`](https://github.com/FNNDSC/pl-med2img) is a wrapper around `med2image` which conforms the CLI usage to a _ChRIS_ ds plugin spec.

[`pfdo_med2image`](https://github.com/FNNDSC/pfdo_med2image/) is a wrapper around `med2image` which generalizes `med2image` so that it can be mapped on all files found via recursive traversal, i.e. analogous to

```bash
find /inputDir -name '*.nii' | med2image ...
```

`pl-med2img`, `pfdo_med2image`, `pl-pfdo_med2img` extend `med2image`. But these need not be separate projects.

The proposal here is to have one repository `med2image` which provides:

- the python library `med2image` (its API will not be changed)
- the script `med2image`, which has the usage and functionality of `pl-pfdo_med2image`
- the CLI options are not much different
- the CLI for `pl-pfdo_med2image` complies to the _ChRIS_ ds plugin spec