Add Eigen to ExternalProject and automatically download if RDK_BUILD_DESCRIPTORS3D

[e-kwsm]

What does this implement/fix? Explain your changes.

Even if RDK_BUILD_DESCRIPTORS3D is ON, 3D descriptors calculators are not built if Eigen is not found.

This PR resolves the dependency.

[bp-kelley]

I think this is a good idea, but it is hard to test as our current builds already have Eigen installed. Perhaps we should add a non-eigen installed entry to the build matrix?

[greglandrum]

LGTM

[greglandrum]

@bp-kelley @ptosco @ricrogz : This PR adds a hard dependency on eigen, but it does download a copy if eigen isn't already installed.

Do any of you see a problem with this?

[ptosco]

I am fine with this. Also, isn't the dependency enforced only if RDK_BUILD_DESCRIPTORS3D is ON? I would be happy with the dependency in any case.

[ricrogz]

I'm also ok with this.

[greglandrum]

Also, isn't the dependency enforced only if RDK_BUILD_DESCRIPTORS3D is ON? I would be happy with the dependency in any case.

Ah, right. Maybe we should just add the dependency globally so that eigen can be used in other bits of the code without concern/complication?

[ptosco]

Ah, right. Maybe we should just add the dependency globally so that eigen can be used in other bits of the code without concern/complication?

The RDKitJS build currently sets RDK_BUILD_DESCRIPTORS3D=OFF, so I probably wouldn't do that.

[greglandrum]

@ptosco : I'm fine with merging this as is.
What's your take?

[ptosco]

Looks good, as the build scripts will look for Eigen3 first, and only in case it is not found and RDK_BUILD_DESCRIPTORS3D=ON it will download Eigen3 from GitLab, otherwise it will use what was previously found (e.g., in /usr/include or in the conda environment).

[greglandrum]

Thanks for the contribution @e-kwsm!