Switch from TestClusterExecutor to FluxClusterExecutor

[jan-janssen]

Currently still requires shutdown(wait=True, cancel_futures=False).

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
__init__.py	0	0	100%
app.py	107	23	78%	51–53, 60–61, 71, 129–130, 141, 144, 147–148, 198, 200, 205–207, 231–233, 260–262
database.py	130	32	75%	69, 98–100, 121–123, 151–153, 165–167, 192–194, 205–206, 208–209, 215, 217–218, 220, 222–224, 238, 261, 265, 283–284
models.py	72	20	72%	37–39, 56, 58, 63–66, 68–72, 74–75, 124–126, 134
visualization.py	93	13	86%	39, 56–59, 148, 152, 177, 179–180, 230–232
worker.py	77	72	6%	23–30, 44, 46, 48–49, 52, 55–56, 58, 60–61, 67–68, 71–72, 74–75, 77–78, 81–84, 87–88, 91–92, 100, 104, 106, 111, 114–117, 120, 125–126, 137, 140–143, 150, 154–157, 160–164, 167–169, 174–175, 188, 190, 192–196
TOTAL	479	160	66%

[ltalirz]

superseded by #121