libgfortran dependency

[awickert]

An user received the error when running GSFLOW v1.2.0:

libgfortran.so.3: cannot open shared object file: No such file or directory

He corrected it through:

sudo apt-get install libgfortran3

I'm not sure if it's just his system, or if this is a more general thing, so I'm first making a note here. I'm also not sure if it's the library or the specific version of the library. If others report this being an issue, I will update the README.md.