Update scaling and fixing parameters

[marcorusc]

This pull request contains the commit necessary to run the ecoli switch model.

1. In intracellular.cpp we have updated the scaling as we discussed in the pre and post update function.
2. We have added a function to check the flux value in the cell
3. We have updated the configuration files with the correct parameters for Oxygen, Glucose and Acetate
4. We have updated the function to color the cells in the SVG file (which will not be needed in the future update as it will be possible to color according to the flux value using the PhysiCell-Studio)

Once accepted this PR, the ecoli model will run smoothly. Adjusting the diffusion of oxygen, you can see that the population on the left grows faster than the one on the right and the latter produces acetate.

[migp11]

Great Marco! thanks