[ENH] Output derivatives to folder outside of BIDS dataset

[tsalo]

References #19. This change will make it easier to track storage of converted data and derivatives separately. It will also make it easy to share just the dataset or just the derivatives, when necessary.

Changes proposed:

• Add PR template.
• Output MRIQC derivatives to parent directory of BIDS dataset directory. For example, if the BIDS dataset is located in /home/data/nbc/project/dset/, then MRIQC outputs will be saved to /home/data/nbc/project/derivatives/mriqc-[version]/ instead of /home/data/nbc/project/dset/derivatives/mriqc-[version]/ (current behavior).

[tsalo]

@mriedel56 @anthonystevendick @adamkimbler Would you all mind taking a look at this at some point before I merge it?

@anthonystevendick I believe that this will affect you most of all. You'll need to move your AHEAD derivatives folder out of your dataset folder before converting new subjects (once this is merged and pulled to the HPC).

[tsalo]

I've also moved the BIDS dataset merging functionality into a separate function in a new module, utils. @adamkimbler would you mind taking a quick look at that to double check the code?