Make Expokit an optional requirement

[schillic]

No description provided.

[codecov-io]

Codecov Report

Merging #1290 into master will decrease coverage by 4.12%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master    #1290      +/-   ##
==========================================
- Coverage   90.41%   86.29%   -4.13%     
==========================================
  Files          63       62       -1     
  Lines        3213     2728     -485     
==========================================
- Hits         2905     2354     -551     
- Misses        308      374      +66

Impacted Files	Coverage Δ
src/ExponentialMap.jl	96.66% <100%> (-3.34%)	⬇️
src/init.jl	85.71% <80%> (-14.29%)	⬇️
src/compat.jl	50% <0%> (-50%)	⬇️
src/Approximations/template_directions.jl	51.79% <0%> (-42.36%)	⬇️
src/Approximations/Approximations.jl	57.14% <0%> (-30.36%)	⬇️
src/Hyperrectangle.jl	54.16% <0%> (-29.84%)	⬇️
src/helper_functions.jl	75.59% <0%> (-17.08%)	⬇️
src/BallInf.jl	84.61% <0%> (-15.39%)	⬇️
src/Complement.jl	85.71% <0%> (-14.29%)	⬇️
src/ZeroSet.jl	86.66% <0%> (-13.34%)	⬇️
... and 48 more

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[mforets]

In the first example of the Introduction now we need to add using Expokit.