__init__.py: Import mesa.experimental

[EwoutH]

Fixes some of the imports in the style of

import mesa
mesa.experimental. ...

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 +1.0% [-0.3%, +2.3%]	🔵 -0.3% [-0.5%, -0.2%]
BoltzmannWealth	large	🔵 -0.4% [-0.8%, +0.1%]	🔵 -0.3% [-1.6%, +1.2%]
Schelling	small	🔵 +0.1% [-0.2%, +0.5%]	🔵 +0.3% [+0.0%, +0.5%]
Schelling	large	🔵 -0.0% [-0.9%, +0.8%]	🔵 -1.0% [-2.1%, -0.1%]
WolfSheep	small	🔵 -0.3% [-0.4%, -0.1%]	🔵 -1.8% [-1.9%, -1.6%]
WolfSheep	large	🔵 -0.4% [-0.8%, +0.1%]	🔵 -1.9% [-2.6%, -1.2%]
BoidFlockers	small	🔵 -2.9% [-3.5%, -2.2%]	🔵 -0.2% [-0.9%, +0.5%]
BoidFlockers	large	🔵 +0.6% [-0.1%, +1.1%]	🔵 +2.1% [+1.7%, +2.5%]