Hager_Zhang first commit

[alisterde]

@MichaelClerx @ben18785 I've got an initial draft of the Hager-Zhang line search algorithm #1160 it is passing all of the tests I've written and appears to be functional. However, I want to write more tests to check specific conditions in the logic before I'll be confident in the algorithm, I also think I need to add a few more convergence checks but I'll address that tomorrow.

I would appreciate any initial thoughts you have about it.

[MichaelClerx]

Maybe we can link to this somewhere in the file. I can never find it :D https://docs.python.org/3.8/reference/expressions.html#yield-expressions

[MichaelClerx]

https://docs.python.org/3.8/glossary.html#term-asynchronous-generator https://docs.python.org/3/reference/compound_stmts.html#coroutines

[alisterde]

This is the paper referenced: https://epubs.siam.org/doi/abs/10.1137/030601880?casa_token=GhCspIQb5DUAAAAA:aVcINgBT6bK9q154vVI9UDxbnPuwNEUCc-Ph_tTVKR6hyfEjkWsk6xNZD6KQMG0ZGH7Xa8XnVCplew