[ENH] Add duecredit module for citations to external software

[ReinderVosDeWael]

The duecredit module allows users to easily find which software they should cite when using BrainStat.

Although this would be a nice addition for all those people whose software we use, this is not a an immediate necessity for the publication of BrainStat. It may also be prudent to wait with implementing this until we have a BrainStat preprint.

On hold until a BrainStat preprint exists.