Bmti error dev

[charliematteo]

Proposed changes

Updated diagonal of inverse cross covariance of the deltaF in density_advanced (used in BMTI) to published version (https://arxiv.org/abs/2407.08094). This includes substantial edits to cython_grads, neighbour_graph and density_advanced.

PLEASE: before accepting pull request see if a fresh install of DADApy cloning this branch succeeds.

Types of changes

What types of changes does your code introduce?3 Put an x in the boxes that apply

• [ x] Bugfix (non-breaking change which fixes an issue)
• [ x ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply.

• [ x ] Lint and unit tests pass locally with my changes
• [ x ] I have added tests that prove my fix is effective or that my feature works