space: Add note that Grids are maintenance only

[EwoutH]

Add a note to the space module level docstring that the Grids are maintenance only.

Also add the PropertyLayer to the classes and remove the Grid, since that's now private (and has been for a while).

Now looks like this:

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 -2.3% [-3.6%, -0.9%]	🔵 +1.0% [+0.7%, +1.3%]
BoltzmannWealth	large	🔵 +0.0% [-0.6%, +0.8%]	🔵 +4.2% [+2.6%, +5.6%]
Schelling	small	🔵 -0.3% [-0.8%, +0.1%]	🔵 +0.3% [+0.0%, +0.5%]
Schelling	large	🔵 -0.9% [-2.2%, +0.2%]	🔵 -1.0% [-3.2%, +1.1%]
WolfSheep	small	🔵 +0.1% [-0.2%, +0.3%]	🔵 +0.3% [+0.2%, +0.5%]
WolfSheep	large	🔵 +0.7% [+0.3%, +1.1%]	🔵 +1.9% [+1.0%, +2.7%]
BoidFlockers	small	🔵 +1.0% [+0.4%, +1.7%]	🔵 +0.2% [-0.4%, +0.9%]
BoidFlockers	large	🔵 +0.7% [+0.4%, +1.1%]	🔵 +0.4% [-0.4%, +1.2%]

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 -1.0% [-2.5%, +0.5%]	🔵 -0.9% [-1.1%, -0.7%]
BoltzmannWealth	large	🔵 -1.1% [-1.5%, -0.7%]	🔵 -0.3% [-1.3%, +0.7%]
Schelling	small	🔵 +0.4% [+0.1%, +0.7%]	🔵 -0.2% [-0.4%, -0.0%]
Schelling	large	🔵 +1.4% [+0.3%, +2.4%]	🔵 +0.0% [-1.4%, +1.4%]
WolfSheep	small	🔵 +1.1% [+0.8%, +1.5%]	🔵 -0.7% [-0.9%, -0.5%]
WolfSheep	large	🔵 +1.3% [+0.9%, +1.6%]	🔵 +0.7% [-0.2%, +1.6%]
BoidFlockers	small	🔵 +0.2% [-0.3%, +0.7%]	🔵 -0.9% [-1.7%, -0.2%]
BoidFlockers	large	🔵 -0.0% [-0.6%, +0.6%]	🔵 +0.0% [-0.6%, +0.8%]

[tpike3]

LGTM

[quaquel]

indeed go ahead and merge it.

One thing I noted but can be done in a separate: remove HexGrid from mesa.space. It has been deprecated already so it can be safely removed.