explainability: new module (#44)

* explainability: new module

    - Add Explainer class and explanation
      data structures to express explanations for feature
      vectors and molecules.
    - Add Christian W. Feldmanns visualization code for shap
      weighted heatmaps of the molecular structure.
    - Add as experimental because we might change the API.
      However, the methodology is fully functional.

molpipeline/experimental/explainability/__init__.py

@@ -0,0 +1,23 @@
+"""Explainability module for the molpipeline package."""
+
+from molpipeline.experimental.explainability.explainer import (
+    SHAPKernelExplainer,
+    SHAPTreeExplainer,
+)
+from molpipeline.experimental.explainability.explanation import (
+    SHAPFeatureAndAtomExplanation,
+    SHAPFeatureExplanation,
+)
+from molpipeline.experimental.explainability.visualization.visualization import (
+    structure_heatmap,
+    structure_heatmap_shap,
+)
+
+__all__ = [
+    "SHAPTreeExplainer",
+    "SHAPKernelExplainer",
+    "SHAPFeatureExplanation",
+    "SHAPFeatureAndAtomExplanation",
+    "structure_heatmap",
+    "structure_heatmap_shap",
+]