Add L-BFGS reconstruction engine

[jfowkes]

Resurrecting #400 as GitHub broke the original PR.

Add Limited-memory BFGS reconstruction engine to as an alternative to ML.

Caveats:

• L-BFGS does not support the object smoothing preconditioner for technical reasons (this would require the ability to invert the Gaussian filter, i.e. exactly de-blur and this is known to be impossible). However probe preconditioning (i.e. probe-object scaling) is supported and this is enough to reconstruct on the moonflower example with a lower error than ML.

[jfowkes]

@daurer this L-BFGS pull request is now ready for review and potential merge.

[daurer]

Needs a CuPy engine before it can be merged. @ptim0626 maybe something for you?

[ptim0626]

A quick comment, the tolerance for LBFGS's tests is increased as it is very likely the discrepancy is a result of numerical instability instead of the problem of implementation.

[jfowkes]

@daurer this PR will soon need to be updated to use the wavefield preconditioner, I'm happy to do this.