[BUG] Rename `hess` field into `hess_inv` for result of BFGS minimization

Also document the fact that the Hessian inverse returned by BFGS is
approximated.

Closes #2647.
Update the interpolative decomposition library to the latest version (attempt two)
DOC: remove backticks on None, False, True
Merge pull request #2686 from sciunto/doc_backticks

DOC: remove backticks on None, False, True
DOC: optimize: warn about approximate hessians
Merge pull request #2659 from dlax/issue-2647

BUG: optimize: Rename `hess` field into `hess_inv` for result of BFGS minimization

Also warn in the documentation that these may be just approximations.
Merge pull request #2658 from inducer/interpolative-update

ENH: linalg: Update the interpolative decomposition library to the latest version