Use Consistent name for when refering to the Engine component

[mrzzy]

Problem

Currently we refer to the Bentobox Engine component as both:

• bentobox-sim, Simulator.
• Bentobox Engine, Engine component.

This makes things confusing as we are using two different terms to refer to the same thing.

• Another possibility for confusion is Simulation that run in the Engine are named the same as the Engine itself.

Proposal

Rename all instances of Sim/Simulator refering to the Bentobox Engine to use 'engine' as the standard term.

• Docker Container/Dockerfile
• sim/ top level folder to engine/
• SIM_ variables in project makefile to Engine-
• GIthub Actions CI mentioning bentobox-sim.