[ITensors] Expose new KrylovKit.realeigsolve features to dmrg

[corbett5]

Description

As of v0.8 KrylovKit supports finding the eigenvectors of real non-symmetric operators with real eigenvalues. This eliminates the need for complex arithmetic. For my problem it produces the same results as the standard hermitian=false algorithm.

I added an option to specify an early return from the eigensolve. I haven't tested this out but the documentation suggests it could significantly improve performance when the initial guess is very good, which I think could be a common case in later sweeps of DMRG so worth trying out.

I also forwarded the eigsolve_verbosity on to eigensolve. Previously this argument was ignored.

How Has This Been Tested?

It has not.

Checklist:

• My code follows the style guidelines of this project. Please run using JuliaFormatter; format(".") in the base directory of the repository (~/.julia/dev/ITensors) to format your code according to our style guidelines.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that verify the behavior of the changes I made.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.

[mtfishman]

I didn't know about KrylovKit.realeigsolve, that's really cool! That's something I've wanted in multiple projects and wasn't even sure if it was possible/practical to do. @JoeyT1994 this is something we were discussing.

I'll take a look at this PR when I get a chance.