Fix Pade approximation and test _exp

[schillic]

In master:

julia> using ReachabilityAnalysis, SparseArrays

julia> using ReachabilityAnalysis: _exp, PadeExpAlg

julia> A = sparse([10.0 0; 0 20]); δ = 0.1;

julia> _exp(A, δ, PadeExpAlg())
ERROR: AssertionError: package `Expokit` is required for this function; do `using Expokit` and try again

julia> using Expokit

julia> _exp(A, δ, PadeExpAlg())
ERROR: AssertionError: package `Expokit` is required for this function; do `using Expokit` and try again