Fides

* Adds new rank 1/2 approximations (BB, BG, PBS, Broyden)
* Adds hybrid hessian approximation schemes (FX, GNSBFGS, SSM, TSSM, HybridUpdate renamed to HybridFixed)
* Adds the possibility to specify residual functions for non-linear least-squares problems, permitting better exploitation of problem properties.

* Changes the default subspace method to 2D by default

* Disable scaled gradient by default
* Change hybrid implementation from predefined iteration to dynamic scheme

* Fix gradient relative tolerance check for negative objective function values
* Updates to README
* Allow disabling scaled gradient steps

* Fix a bug where lanczos method would yield complex optimization variables
* Update readme

* use lanczos method to compute better descent solution if newton is not a descent direction (2D subspace minimization only)
* add additional safeguards for step acceptance and tr update
* add additional scaled gradient to stepback strategy
* add steinhaugs cg method as subspace solver
* fix 1d trust region analytical solution
* update documentation (thanks to dweindl)