EP: Reorganize the org's packages

Aim: This enhancement proposal is to reorganize the packages orgs surrounding reachable set computations.

(this is a draft)

Steps:

1- move set-based algorithms to SetBasedRecurrences.jl

2- rename Reachability.jl to ReachabilityAnalysis.jl

3- do MathematicalSets.jl

4- do MathematicalPredicates.jl

5- better docs for reachability analysis (essential)

6- add these packages in the general registry

The rest of this document is about point 1.