Set sim seed from network if sim seed not given.

[jgosmann]

Motivation and context:
Issue #980 hasn't gotten any attention in a while, but apparently the last decision was to implement the behavior as in this PR, i.e. to set the simulator seed from the network seed if no simulator seed is given. Still seems like a good idea to me because today I was again surprised that my network with a process was behaving non-deterministic despite a network seed being set.

Fixes #980.

Interactions with other PRs:
none

How has this been tested?
added a test

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• Bug fix (non-breaking change which fixes an issue)?
• New feature (non-breaking change which adds functionality)?

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• I have run the test suite locally and all tests passed.

[jgosmann]

Merry Christmas! 🎄

Can we get this PR merged? I once again lost a day trying to figure out why I get different results despite identical (network) seeds.

[tbekolay]

Merry Christmas to you as well! LGTM

[jgosmann]

Thanks! ❤️