Add backend option to exp in discretization

[mforets]

[schillic]

An alternative is to use the functionality for controlling the backend through LazySets.

julia> ReachabilityAnalysis.LazySets.set_exponential_backend!(ExponentialUtilities)
Val{:ExponentialUtilities}()

julia> ReachabilityAnalysis.LazySets.set_exponential_backend!(Expokit)
Val{:Expokit}()

I have prepared #704.

The only thing to consider is to add the function set_exponential_backend! to ReachabilityAnalysis so you do not have to write this long command. Note that LazySets does not export the function.