Fix optional packages in Exponentiation module

[schillic]

• fix the warning below, which required to load the init function inside the Exponentiation module
  • comment out _expv because it was not used anywhere

WARNING: Method definition _exp(SparseArrays.SparseMatrixCSC{Tv, Ti} where
Ti<:Integer where Tv, ReachabilityAnalysis.Exponentiation.PadeExpAlg) in module Exponentiation at
/home/runner/work/ReachabilityAnalysis.jl/ReachabilityAnalysis.jl/src/Discretization/Exponentiation.jl:139
overwritten in module ReachabilityAnalysis at
/home/runner/work/ReachabilityAnalysis.jl/ReachabilityAnalysis.jl/src/Discretization/Exponentiation.jl:143.

• change require(@__MODULE__, Expokit) to the correct form require(@__MODULE__, :Expokit)
• remove FastExpm because it was not used anywhere