UX: Simplify schedulers class names

[rht]

What if, instead of

from mesa.time import RandomActivation
self.schedule = RandomActivation(...)

we do

import mesa.time
self.schedule = mesa.time.Random(...)
# or, as per https://github.com/projectmesa/mesa/issues/1229
self.schedule = ma.time.Random(...)

As in, the context that it is a scheduler, is reflected by the namespace, and so we don't need to redundantly say that it is an "activation":

• StagedActivation -> Staged
• RandomActivationByType -> RandomByType
• and so on.

This is inspired by the way Agents.jl name their schedulers.

[rht]

We may need to rename mesa.time to mesa.scheduler for clarity.