Restructure project so we have backend as one submodule which contains job manager and simulator

[nbarlowATI]

Goal

Allow integration testing between tightly coupled job manager and simulator without relying on middleware

Acceptance criteria

docker-compose file running pytest

Out of scope

integration with middleware

Implementation notes

• gateway-job-manager-openfoam and gateway-simulator-openfoam become submodules of gateway-openfoam.
• do this in "backend-restructure" branch of gateway.

[nbarlowATI]

Done