First release of ExtSym code

As described in the readme, this is to open up the source code for the ExtSym program, which is also described here: http://www.markvardsen.net/projects/ExtSym/main.html .

It is the complete source code for ExtSym except it does not ship the Numerical Recipies library.

An executable of the ExtSym program can be downloaded from http://www.markvardsen.net/projects/ExtSym/raw-download.html, and the code here matches that of the executable: version 2008 (update V1).

I am creating this release partly to test creating a DOI with Zenodo, and give anyone the option to site this DOI, should they copy parts of the source code of ExtSym and would like to reference this.

And here is the Zenodo DOI: