Clean up ARCH folder

[schillic]

We usually only need the X.jl and X_benchmark.jl files for each model X.

[mforets]

Just throwing an initial idea (modify here):

Filename	Purpose	Returns
model.jl	Defines the model in an algorithm-agnostic way.	An initial-value problem P and general options O.
model_ALGORITHM.jl	Everything to run solve for a given algorithm. Loads model.jl	Algorithm-specific options O_algorithm.
model_benchmark.jl	Adds the model to the benchmark suite for each algorithm.