Mritopng

This version contains the ability to apple **auto-contrast** when converting the DICOM files. This provides the following benefits

* Ensure a more uniform output across files when converting multiple files
* Match the processing of commercial processors a little more closely

The behavior is backward compatible. To enable auto-contrast, you need to add either the `-c` or `--auto-contrast` flag when running the command.

This ability is also added to the functions as an optional paramenter which defaults to `False`

This fix addresses an issue where the input DICOM files might have readings below 0. In the example files provided by a user here, A value of -1024 is seen for the background.

I've modified the logic to simply clip such values to 0.