Enable CMake package installation

[johnastratton]

Major problem, will require essentially redesigning the build system, but necessary if we want customization to actually be supported.

[johnastratton]

Somewhat resolved - example repository shown at https://github.com/johnastratton/DENSE_multimodel_example, however that only works when cloning the repo. It doesn't work if you tried to use CMake to install DENSE and then find it as a package in another CMake project.