Adding JSSP environment

[DavideTr8]

Description

Added the JSSP env from Learning to Dispatch.

Motivation and Context

We wanted to have a JSSP environment from a published paper.

• [ ] I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• [x] New feature (non-breaking change which adds core functionality)

Checklist

Go over all the following points, and put an x in all the boxes that apply. If you are unsure about any of these, don't hesitate to ask. We are here to help!

• [ ] My change requires a change to the documentation.
• [x] I have updated the tests accordingly (required for a bug fix or a new feature).
• [ ] I have updated the documentation accordingly.