cell space: Add convenience properties for grid width and height

[quaquel]

90% of all use cases of grid will be 2D so it is convenient to have easy access to the width and the height of the grid.

While porting the examples over to the experimental grid spaces, I frequently need to write self.grid.dimensions[0] etc. This makes it a bit more readable because I can just do self.grid.width etc.

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 -0.3% [-1.5%, +0.8%]	🔵 -0.2% [-0.3%, -0.0%]
BoltzmannWealth	large	🔵 -1.8% [-2.2%, -1.4%]	🔵 -1.5% [-2.1%, -0.9%]
Schelling	small	🔵 -0.2% [-0.7%, +0.3%]	🔵 -0.7% [-1.0%, -0.5%]
Schelling	large	🔵 +0.0% [-1.6%, +1.5%]	🔵 -0.7% [-1.5%, +0.2%]
WolfSheep	small	🔵 -0.5% [-0.7%, -0.3%]	🔵 -0.9% [-1.1%, -0.7%]
WolfSheep	large	🔵 -0.4% [-1.2%, +0.4%]	🔵 +0.7% [-0.5%, +2.0%]
BoidFlockers	small	🔵 +1.1% [+0.6%, +1.6%]	🔵 +0.1% [-0.5%, +0.7%]
BoidFlockers	large	🔵 +0.3% [-0.2%, +0.7%]	🔵 +0.8% [+0.3%, +1.4%]

[EwoutH]

Nice, we discussed the API before, it clean and convenient.

Thanks!