solve_shifted_system! method for LBFGS solving step in recursive way

[farhadrclass]

Computes the regularized L-BFGS step by solving the linear system

Include:

1. new method
2. unit tests
3. edit readme to include the name

Before merging, we have run the following:

1. allocation and benchmark tool ran to make sure no allocation
2. unit tests with different sizes and accuracy

TODO:

1. GPU support

Ref:
@misc{erway2013shiftedlbfgssystems,
title={Shifted L-BFGS Systems},
author={Jennifer B. Erway and Vibhor Jain and Roummel F. Marcia},
year={2013},
eprint={1209.5141},
archivePrefix={arXiv},
primaryClass={math.NA},
url={https://arxiv.org/abs/1209.5141},
}

[dpo]

Thank you.