Allow other entities than PASEOS to use event loop

[johanos1]

Feature

Desired Behavior / Functionality

Current functionality: PASEOS decides when the async event loop is started and shut down. In particular, when an activity is performed, the async event loop is started and when the activity is closed, the event loop is too.

However, there may be other entities that also need access to the event loop, e.g., the communications layer that requires an event loop to be running in the background all the time. There are multiple issues with the current way of using async in PASEOS:

1. If no activity is running and the communications layer is launched, the event loop is created from the communications layer and a RunTimeError "asyncio.run() cannot be called from a running event loop" occur.
2. If the communications is launched as an activity, PASEOS will execute the task forever as the communications loop will not end. This will effectively block any other activity from being started.
3. If the communications is forced to end, all the bells and whistles must be re-launched every time we want to perform communication in an activity.

Desired functionality: External entities should be able to access the event loop.

Here is a minimal example to reproduce the error:

import asyncio
import pykep as pk
import paseos
from paseos import ActorBuilder, SpacecraftActor

async def start_network_stuff():
    while True:
        await asyncio.sleep(1)
        print("outside job")

async def main_loop(sim):
    while True:
        # Start up network engine
        asyncio.gather(start_network_stuff())

        async def paseos_job(arg):
            await asyncio.sleep(1)
            print("paseos job")

        sim.register_activity("Job", paseos_job, power_consumption_in_watt=0)
        sim.perform_activity("Job")


earth = pk.planet.jpl_lp("earth")
sat = ActorBuilder.get_actor_scaffold(
    "sat1", SpacecraftActor, [10000000, 0, 0], pk.epoch(0)
)

sim = paseos.init_sim(sat)

asyncio.run(main_loop(sim))

What Needs to Be Done

Discuss potential changes in the architecture to allow for external usage of the event loop.

How Can It Be Tested

Via the above code for example and also via the already existing tests.