Various sign/indexing fixes and performance improvements (#2)

* - made changes that agree with the Julia version (analytic Exchange matrix elements) up to flipping b field sign (complex conjugate and a sign).

* Did checks and compared Julia implementation results to revised package results

* added additional comments

* fix: Update README to include sigma notation for matrix elements and exchange kernels

* added sigma feature to planewave part
- next to do the same for exchange matrix element

* fix: Update execution counts and correct sigma parameter in Checks_exchange_pwme notebook

* fix: Remove unused code cell from Checks_exchange_pwme notebook

* Edited Exchange matrix elements to include boths sign of sigma (B_z * charge carrier).
- Added h5 file that has planewave and exchange elements for randomly sampled G points

* Refactoring and performance improvements

* fix: Correct LaTeX formatting for magnetic field sign in README

* fix: Add missing line breaks in wavefunction section of README

* fix: Add Sparsh Mishra as an author in CITATION.cff and pyproject.toml

* Initial plan

* Initial plan

* Initial plan

* Enhance docstring for get_exchange_kernels_GaussLegendre function

Co-authored-by: skilledwolf <18141588+skilledwolf@users.noreply.github.com>

* Remove commented-out code and placeholder comments from exchange_hankel.py

Co-authored-by: skilledwolf <18141588+skilledwolf@users.noreply.github.com>

* Initial plan

* Address code review feedback on docstring clarity

Co-authored-by: skilledwolf <18141588+skilledwolf@users.noreply.github.com>

* Add sign_magneticfield validation to exchange_legendre.py

Co-authored-by: skilledwolf <18141588+skilledwolf@users.noreply.github.com>

* Add missing blank line before Notes section header

Co-authored-by: skilledwolf <18141588+skilledwolf@users.noreply.github.com>

* Initial plan

* Address PR review comments: trailing whitespace, validation, docstring, grammar, unused variable

Co-authored-by: skilledwolf <18141588+skilledwolf@users.noreply.github.com>

---------

Co-authored-by: sparsh mishra <sparshmishra1@gmail.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: skilledwolf <18141588+skilledwolf@users.noreply.github.com>

Release 0.1.0