Move cell movement into a descriptor

[quaquel]

This PR is for discussion in relation to #2292. It shows how it might be possible to use composition for cell-related movement by encapsulating all the movement behavior into a descriptor. So instead of having a special Agent.move_to method, you can do Agent.cell = new_cell. You can also use whatever name you want instead of cell.

An additional benefit that I had not realized at first is that it cleans up the API a bit more as well. A user does not have to interact with the Cell class directly. She only needs to assign the Cell instance to whatever attribute is used for the descriptor.

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 -1.7% [-2.7%, -0.8%]	🔵 -0.2% [-0.4%, +0.0%]
BoltzmannWealth	large	🟢 -3.6% [-4.1%, -3.1%]	🟢 -9.2% [-10.4%, -8.1%]
Schelling	small	🔵 -0.2% [-0.7%, +0.3%]	🔵 -2.2% [-2.9%, -1.5%]
Schelling	large	🔵 +0.3% [-0.4%, +1.2%]	🟢 -13.7% [-15.5%, -12.2%]
WolfSheep	small	🔵 -0.8% [-1.1%, -0.4%]	🔴 +7.9% [+7.4%, +8.4%]
WolfSheep	large	🔵 -0.2% [-1.6%, +0.8%]	🔵 -0.1% [-2.4%, +2.5%]
BoidFlockers	small	🔵 -0.4% [-1.0%, +0.0%]	🔵 -1.3% [-2.1%, -0.5%]
BoidFlockers	large	🔵 -0.6% [-1.2%, -0.1%]	🔵 -2.1% [-2.6%, -1.5%]