Add "basic" backtrack line search

[eparish1]

We need to add a basic backtracking search that only allows a step if the residual goes down, | r(x_{k+1} ) | \le | r(x_k) |.

[mzuzek]

Hi @eparish1, finally we can have $|r(x_{k+1})| \le |r(x_k)|$ in GitHub markdown now :-) (just wrap your LaTeX math with $ or $$)