There is not much different between the cudatoolkit
expressions:
pkgs/development/compilers/cudatoolkit/5.5.nix
pkgs/development/compilers/cudatoolkit/6.0.nix
pkgs/development/compilers/cudatoolkit/6.5.nix
This commit removes those and replaces them with
pkgs/development/compilers/cudatoolkit/generic.nix
and adds cudatoolkit version 7