- Changed the name of the `tspex_plot` method to `plot_histogram`.
- Use the `constrained_layout` argument in the `plot_histogram` method.
- Added the `plot_heatmap` method.
- Raise an error in case the user tries to create a `TissueSpecificity` object with a matrix containing negative values.
- Deprecated the `to_file` method in favor of using the native pandas methods.