Update createNewMetaboliteStructure.m

[farid-zare]

Invalid MATLAB variable names was making errors. It is solved now by generating valid MATLAB variable names. Documentation is enhanced.

I hereby confirm that I have:

• [X] Tested my code on my own machine
• [X] Followed the guidelines in the Contributing Guide
• [X] Selected develop as a target branch (top left drop-down menu)

(Note: You may replace [ ] with [X] to check the box)