Install doc is out of date

[deitrr]

The doc file install.rst contains directions for my original setup and is now out of date with the current installation method. Took me a little while to figure out how it works now, and I am not sure I fully understand how it works. Would one of you mind updating this?

[deitrr]

Followup: I see now that vspace is automatically installed along with vplanet. So I guess the issue is more about how to manage the repository as developers. We should update the docs in this repository to explain how to install vspace as a user (via the installation of vplanet) and as a developer of vspace (which I haven't figured out how to do yet, LOL)