Make sure polyfun submodule gets installed

[bschilder]

The polyfun repo is not getting included in "inst/tools" when installing echofinemap.
This is because the submodule is being interpreted as an empty folder by R:
https://stackoverflow.com/questions/50474445/how-could-i-release-an-r-package-on-github-using-github-submodules

Suggested solution with packrat:
https://blog.methodsconsultants.com/posts/using-packrat-with-git-submodules/

Discussion on devtools:
r-lib/devtools#1163
r-lib/devtools#1222

git2r package:
https://github.com/ropensci/git2r

This function seems promising:
r-lib/devtools#751 (comment)

[bschilder]

Intermediate solution: when users request to use PolyFun for the first time, they will have the option of re-installing echofinemap to add PolyFun. Not ideal, but it does work pretty consistently.