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* Refactored external tools into the ``imfusion.external`` module.
* Use docker/tox for testing against multiple Python versions locally.
* Added additional checks for inputs and improved error messages.
* Added support for DataFrame insertion inputs to DE testing functions.
* Added building of exon gtf as part of imfusion-build.
* Added identification of endogenous fusions using STAR-Fusion as part
of imfusion-insertions (using STAR). Also adds script for building
(murine) STAR-Fusion references.
* Made matplotlib/seaborn lazy imports that are only required when actually
using the plotting functions. This makes IM-Fusion easier to use on
headless servers/HPCs.