Rework Barzilai-Borwein rule to be able to use it as an initial guess for other line searches

[mateuszbaran]

It could be better than 1.0 in other cases.

[kellertuer]

I do not fully remember, what would this rework need, so what needs to be done more precisely?

[mateuszbaran]

Initialization needs to be separated out to a new class that subtypes AbstractInitialLinesearchGuess like HagerZhangInitialGuess. Right now the Barzilai-Borwein rule is integrated into NonmonotoneLinesearchStepsize.

[kellertuer]

Ok, thanks. Then I have to look closer on what you did with that Initialguess type, I either did not read that carefully enough when you introduced it – or forgot it since then ;)