[WIP] Setting the PPM advection problem

[aisclark91]

This PR, implements:

• The PPM mesh reconstruction in the interface of the centered state.
• The advection PPM numerical scheme.
• An example on which the PPM numerical scheme operates on.
• A Test that provides the validity of the numerical scheme.

Two more things remain:

• To clean simulation.py and inherit as much as possible, and make soft links to the original advection problem folder.

[aisclark91]

I decided to clean the previous PR, since it contained many forced pushes. (I don't like the idea of pushing something that probably I shouldn't had). The advection PPM method now converges at second order as follows:

[aisclark91]

Comparing the PPM vs PLM method, I have decided to compare the smooth problem using both methods, giving the following convergence result:

[aisclark91]

This PR is ready for review. One changed proposed by @simonguichandut was to include the PPM as an option in the advection problem, leaving different folders for different time integration schemes. I want to follow this approach in a separate PR.