Use git subtree for pymie

[vnmanoharan]

We should separate out the Mie scattering code into its own python package, and then include it as a git submodule in the structural-color repository. The new Mie package might be useful for other calculations besides those in structcol.

[vnmanoharan]

Code is now separated in mie-separated branch. Currently using structcol requires installing the python-mie package.

In the future, we might want to consider using git subtree to include the pymie code into the structcol repository, while maintaining the history. Above I recommended git submodule, but it's not easy to get that to work with python packages containing a root directory with a setup.py file, since the extra directory makes it difficult to import the package. git subtree would allow us to directly incorporate only the module subdirectory (python-mie/pymie) into the structcol directory. Here are a couple of posts about using git subtree with subdirectories:

https://jrsmith3.github.io/merging-a-subdirectory-from-another-repo-via-git-subtree.html

https://stackoverflow.com/questions/23937436/add-subdirectory-of-remote-repo-with-git-subtree

And one on git subtree vs git submodule: http://candidtim.github.io/git/2013/10/20/git-subtree.html