Custom contraction

[ryanmrichard]

Is this pull request associated with an issue(s)?
No

Description
Implements a custom contraction kernel for Eigen Tensor.

TODOs
None. R2g if it passes CI.

[ryanmrichard]

@jwaldrop107 I tested this with your sigma PR and it seems to work (as long as the eigen_contraction.cpp test doesn't use Sigma since this PR bypasses that function).

[ryanmrichard]

After thinking about it some more this morning I should move this under the eigen_contraction API. In particular, I'm not sure this is actually getting thoroughly tested because I think proving that contraction works fell to that function. I'll work on making that change today.

[ryanmrichard]

@jwaldrop107 once this passes CI this should be ready. FWIW, I tried this PR with your Sigma branch and your Sigma changes seem to work now. Point being I'm relatively confident it's a bug in Eigen's contraction routine that was causing the error.

[jwaldrop107]

Works when I tested in combination with the add_sigma branch. LGTM.

[jwaldrop107]

🚀 [bumpr] Bumped! New version:v0.0.41 Changes:v0.0.40...v0.0.41