Simplify overapproximation option in BFFPSV18

[mforets]

get_overapproximation_option in the discrete post is there to make the transformation from template directions symbols (alias) to arguments that are accepted by LazySets overapproximate. In JuliaReach/LazySets.jl#1329 it is proposed to move that part directly to LazySets. Then get_overapproximation_option can be dropped with the advantage that future updates in LazySets overapproximate are available.

In the meantime, PR #627 allows to use template dirs in the discrete post by adding more checks.

[schillic]

In #633 I remove these symbols for simplicity.