Remove mesa.flat namespace

[rht]

This doesn't seem to be used as often as the current simple namespace.

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
Schelling	small	🔵 +0.8% [+0.6%, +1.0%]	🔵 -0.8% [-1.1%, -0.6%]
Schelling	large	🔵 +1.3% [+0.6%, +1.9%]	🔵 -4.3% [-6.0%, -2.5%]
WolfSheep	small	🔵 -0.3% [-0.9%, +0.4%]	🔵 -0.8% [-0.9%, -0.6%]
WolfSheep	large	🔵 -0.5% [-1.0%, -0.1%]	🔵 -3.2% [-5.5%, -1.2%]
BoidFlockers	small	🔵 -0.1% [-0.7%, +0.5%]	🔵 +1.8% [+1.2%, +2.4%]
BoidFlockers	large	🔵 -0.1% [-0.8%, +0.5%]	🔵 +0.6% [-0.1%, +1.1%]

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
Schelling	small	🔵 -0.6% [-1.0%, -0.3%]	🔵 -2.0% [-2.5%, -1.5%]
Schelling	large	🔵 -0.2% [-0.9%, +0.6%]	🔵 -2.5% [-5.6%, +0.8%]
WolfSheep	small	🔵 -0.0% [-1.0%, +1.0%]	🔵 -1.2% [-1.5%, -1.0%]
WolfSheep	large	🟢 -6.0% [-7.0%, -4.9%]	🟢 -19.7% [-21.6%, -17.6%]
BoidFlockers	small	🔵 -0.6% [-1.2%, +0.2%]	🔵 -0.1% [-0.8%, +0.5%]
BoidFlockers	large	🔵 -0.6% [-1.1%, -0.2%]	🔵 +0.1% [-0.6%, +0.9%]

[EwoutH]

Agreed, but I'm afraid this is a 3.0 ticked since it's breaking.

[EwoutH]

2.3 is released, so we can start with 3.0 development work, including moving this PR forward!

Maybe we should add a deprecation warning to 2.3.x though.

[rht]

Can this be merged ASAP? This is a straightforward PR.

[EwoutH]

Can we add a deprecation message to the 2.3 branch?

[rht]

I don't think anyone has ever used this namespace. It's not documented, and was already voted out in the past.

[EwoutH]

But adding a deprecation warning should be straightforward and is best practice.

[rht]

Needs to fix the CI tests ASAP. The deprecation warning is not going to be a part of this PR. See #2133, which fixes the tests.