San Diego DME Reactions 2015/07/31

The DME chemistry is presented here separately from the main mechanism.
To include these reactions into the computations, one must add them
to the main mechanism file.

The DME reactions have been introduced based on the latest work by 
J. C. Prince and F.A. Williams as per the following publication - 
J.C. Prince, F. A. Williams. "A short reaction mechanism for the combustion of 
dimethyl-ether. Combustion and Flame", 2015. DOI: 10.1016/j.combustflame.2015.06.016