Manuscript figures and variants

[bmusangu]

## Summary

This branch adds the files required to reproduce the manuscript figures from a clean repository checkout.

The original request was to create a branch containing the code used to generate the figures. In addition to the plotting scripts, I included the corresponding GPFA variants and packaged benchmark results to support end-to-end code reproducibility.

## What is included

- Manuscript figures in PNG format
- Plotting scripts for Figures 1 and 2
- Packaged result files used by the plotting scripts
- Command-line interfaces for regenerating the benchmark results
- Instructions for running the experiments and plotting workflows
- Structured citation metadata in `CITATION.cff`

### Figure 1

- Dense linear-algebra GPFA variant
- Block-persymmetric GPFA variant
- Synthetic profiling experiment
- Packaged synthetic and neural profiling results
- Script for regenerating the combined manuscript figure

### Figure 2

- Exact-versus-sparse GPFA benchmark
- Sparse GPFA implementation
- Packaged result files covering inducing-point counts from 5 through 320
- Script for merging the result files and regenerating the manuscript figure

## Why the GPFA variants are included

The initial request focused on the code that generates the figures. I also included the GPFA variant implementations used to produce the underlying results.

This makes the branch more useful for reproducibility because readers can inspect and run the methods being compared, rather than only regenerate figures from precomputed results.

If these implementations are considered outside the intended scope of this branch, they can be removed while retaining the plotting scripts and packaged results.

## Reproducibility

The workflows are self-contained within the repository:

- Plotting scripts use packaged result files by default.
- Input and output paths can be overridden through command-line options.
- Reduced smoke-test configurations are documented for the computationally expensive experiments.

See `manuscript_assets/README.md` for setup and execution instructions.

## Additional cleanup

- Added `CITATION.cff` for GitHub and Zenodo metadata. Please update this with relevant details.
- Standardized author names across authoritative metadata.
- Centralized the project copyright notice in `LICENSE.txt`.
- Corrected the Sphinx release value from `0.1.0.` to `0.1.0`.
- Updated the preprocessing example to import from `blockinvgpfa`.

[jdrugo]

Thanks for the PR! Right now it packs too many changes into a single PR. I suggest splitting it into (at least) two PRs:

• The first should include general changes that also apply to the main branch and are independent of the manuscript assets. This includes removal of the per-file copyright notices, and addition of CITATION.cff. These changes can be made to the main branch.
• The second should contain changes that only concern the manuscript assets, that is, everything that is currently in the manuscript_assets folder. This changes should be in a separate branch, and version number in docs/conf.py and CITATION.cff should be bumped to 0.2.0.

Please also update the year for the copyright notice in LICENSE.txt.

[bmusangu]

Let's keep the branch temporarily until both new PRs have been reviewed and their contents verified. After both new PRs are merged, we shall delete manuscript-figures-and-variants from GitHub.