Update CMakeLists.txt with bin install

[VictorEijkhout]

The executable was not being installed to the bin directory.
I added a install line.

[jmstone]

Victor: thanks for the contribution. I am a cmake newbie. Where is the bin directory located? Does cmake create it automatically? When I run cmake with this change on my laptop the executable still appears in build/src/

[VictorEijkhout]

the bin directory goes in the CMAKE_INSTALL_PREFIX. you need to do

1. cmake -D CMAKE_INSTALL_PREFIX=/home/you/installations/athena <other arguments> or whatever location
2. make (that leaves everything in build)
3. make install that puts the bin directory in the install prefix.