Clean up install requirements

[jwodder]

Some of the packages listed as requirements were not actually in use, and there were some other packages used by pynidm that weren't listed.

Specifically:

• duecredit was unused
• graphviz (the Python project, not to be confused with actual Graphviz) was unused
• orthauth was unused
• pydotplus was unused
• python-dateutil was used but unlisted; the code only worked because it was installed by another dependency
• rdflib-jsonld has been subsumed into rdflib
• scipy was used but unlisted; the code only worked because it was installed by another dependency
• matplotlib and seaborn were unlisted, not installed by anything, and only used by a few files in nidm/experiment/tools/, none of which are entry points for the project; I decided to just list them as extras.

[yarikoptic]

• duecredit was unused

it might be unused directly, but having it installed guarantees that iff users run with DUECREDIT_ENABLED, then they would get citations. But indeed we better re-add it with corresponding reference (https://f1000research.com/articles/11-228 ? or there is a JOSS paper on "The Neuroimaging Data Model (NIDM) Linear Regression Tool" contained in here).

[yarikoptic]

ok, let's proceed , thank you for combing them all up

[github-actions]

🚀 PR was released in v4.0.0 🚀