[Design] Simplify class names

[rht]

Agents.jl has a short naming scheme for its schedulers, like Schedulers.by_id / Schedulers.random, which relies on the module name to explain more what it does, instead of a more standalone name like by_id_activation. See https://github.com/JuliaDynamics/Agents.jl/blob/b479c1460250f6c74bdd62acd0cf059a222a9c67/src/submodules/schedulers.jl#L36-L52.

Wouldn't it be nice if the schedulers in Mesa are called this way? E.g.

• RandomActivation -> schedulers.Random
• SimultaneousActivation -> schedulers.Simultaneous
• StagedActivation -> schedulers.Staged