Unable to install exoplanet 0.4

[emilygilbert]

When trying to install exoplanet from source following the instructions for installing from source with python -m pip install -e . I get the following error:

In file included from src/exoplanet/theano_ops/lib/include/exoplanet/starry/ellip.h:21:
src/exoplanet/theano_ops/lib/include/exoplanet/starry/utils.h:13:10: fatal error: 'Eigen/Core' file not found
#include <Eigen/Core>
         ^~~~~~~~~~~~
1 error generated.
error: command 'x86_64-apple-darwin13.4.0-clang' failed with exit status 1

@mrtommyb is getting the same error as well

Setup:

• exoplanet: 0.3.3, but trying to install 0.4
• MacOS Mojave, 10.14.6
• Python 3.8.3

[dfm]

The git clone needs a --recursive flag or you can run

git submodule init
git submodule update

to download the Eigen submodule.

Pull requests welcome to fix the docs :D