Allow individual directory names in database to be specified

[paulromano]

Description

Normally directory names in the database are selected at random with the uuid module. However, in some cases users may wish to specify the directory names. This PR adds a new output_dir argument on Plugin.__call__ that enables the relative directory path to be specified.

Checklist:

• My code follows the style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• I have updated the CHANGELOG.md file (if applicable)
• I have successfully run examples that may be affected by my changes

[nstauff]

Thanks @paulromano - I reviewed and tested and it works fine. One thought - would it make sense to add an option to return the file to main directory? I think this would be useful to some users.

[paulromano]

Returning files to the main directory would break the result/database functionality since there is an assumption that a single result is stored in its own directory. If we were to do that, the only way I could imagine it working is to simply not save such a result to the database, which may also be confusing to a user if they're expecting to be able to retrieve that result later. In any case, if we were to consider such a change, it would likely be substantial and require a new PR.