doc-func: change some variables in function MissingTransDeadEnd to clarify

[feiranl]

Main improvements in this PR:

change some variable names and clarify meaning of each variable

I hereby confirm that I have:

• Tested my code with all requirements for running the model
• Selected devel as a target branch (top left drop-down menu)
• If needed, asked first in the Gitter chat room about this PR

[BenjaSanchez]

Thanks for the PR Feiran, it does help to understand better. A couple lingering questions/observations are:

1. Why is DEM_reason bigger than DEM? Should't they be of the same size?
2. Note that even though you are trying to save at the end DEM_maysolve and DEM_nosolve, as you are not returning them as output they are not stored when the function is called.

[feiranl]

1. Becasue there may be more than one way to fix the gap.
   For example: there are one met appears in three compartmets: A[c] A[m] A[er]
A[c] and A[m] are not deadend mets, A[er] is the deadend met. There are two ways of dismiss the deadend of A[er] by adding either one transport rxns from A[c] and A[m]. When you unique the first column of DEM_reason, you will get the same number with DEM.

2. Yes, Only the canbesolved will be evaluated manually at the end. Therefore, in order not to return so many variables causing confusion, so I combined DEM_maysolve and DEM_nosolve toghther to be DEM_reason. If people are interested, they can check this variable to see the reason/possible solution. I don't delete the variable at the end, because sometime I need to check the consistency of canbesolved with DEM_maysolve.

[BenjaSanchez]

1. Thanks for clarifying it here :)
2. What I mean is that this line is redundant, because as MissingTransDeadEnd is a function, the only variables that will be returned are model, canbesolved, DEM and DEM_reason. Everything else will be cleared by default.