Add the legacy EGS_Windows (BEAMnrc viewer) source code to the repository

[ftessier]

The EGS_Windows source code has not been available to download from NRC since May 2013. Users are periodically asking about this gui to view BEAMnrc geometries and tracks. I posted answers to common questions in #783. I also added the standalone executable to the assets for Release 2021, for convenience.

I think we ought to add the EGS_Windows source code to the repo, so that people can compile it, and perhaps develop it further (or use some of the code to build another viewer...). At any rate, adding these source files to the repo implies cleaning the file headers and adjusting the licence to move to AGPL like EGSnrc itself. We also have the code that @mchamberland curated to be able to compile EGS_Windows on macOS, which can help us update the makefiles to improve compilation on modern linux distributions.