Inexact Newton Steps in Levenberg Marquardt

[avik-pal]

See the last part of http://ceres-solver.org/nnls_solving.html#levenberg-marquardt. This is basically already there and this is mostly a internal solver selection mechanism:

1. Add a forcing function to LM
2. Add a newton_step = Val(:exact) / Val(:inexact) option. Set it to nothing by default
3. Default to exact for factorization, else inexact
4. Inexact Newton is just setting the tolerance correctly in dolinsolve.

[avik-pal]

https://www.sciencedirect.com/science/article/pii/S0377042705007958

[avik-pal]

With the redesign this is very easy to do now, we need to add forcing terms to the newton-style descent algorithms and all algorithms will be able to use it