Revise return types in concrete ops

[mforets]

• Concrete projection with project

Applies to <:AbstractLazyReachSet.

Current:

input	output
set, <:LazySet	lazy set
reach-set, <:AbstractLazyReachSet	lazy set
flowpipe, <:Flowpipe or <:HybridFlowpipe	vector of lazy sets
solution, ReachSolution{<:Flowpipe} or ReachSolution{<:HybridFlowpipe}	vector of lazy sets

Not sure to keep this though. In some circumstances, it is more convenient to keep the structures (as in the overapproximation case from below).

• Concrete overapproximation with overapproximate

input	output
set, <:LazySet	lazy set
reach-set, <:AbstractLazyReachSet	reach set
flowpipe, <:Flowpipe or <:HybridFlowpipe	flowpipe
solution, ReachSolution{<:Flowpipe} or ReachSolution{<:HybridFlowpipe}	solution

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I think it's convenient for the concrete projection of a flowpipe given a projection matrix M to return a flowpipe: https://github.com/JuliaReach/ReachabilityAnalysis.jl/pull/145/files.

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• [x] Convenience function for the concrete projection of a flowpipe
• [ ] Tests
• [ ] Other flowpipe types
• [ ] Reach-sets