Revise discretize.jl

[schillic]

After solving #41, we may want to do some refactorings:

• Outsource common parts (e.g., the matrix computation) to helper functions.
• Can we split and determine some functions by types rather than by if-then-else chains?
• Outsource the file to a different module? (It is used in ReachSets and Properties.) (outdated)
• Some data is computed several times (e.g., exp(δ*Anorm) - 1. - δ*Anorm). Avoid this.
• Add an option for trying to invert the matrix in the interpolation method (instead of the 3n×3n matrix computations). see Add an option to discretization for computing the matrix inverse #514

[mforets]

• I think that the pull request Revise discretize #503 addresses points 1 and 4.
• Points 2 is half-addressed since it does not completely remove type checks in the code to handle the inputs, instead of using dispatch -- but i think it is good as is :)
• Point 3 is not addressed, discretize still lives in the ReachSets module -- i don't have any particular suggestion here. We could move this file upwards to the root src/discretize.jl..
• Point 5 is interesting and not addressed in Revise discretize #503; in general one can try to invert the matrix P and obtain Phi1 and Phi2 without exponentiation. I don't have an intuition about the outcome but i propose to make a new issue for experiments.