Version 0.4.1

• Update Nix build to 21.05 #561
• New documentation design #559
• Better API documentation #543
• Use Nix to Build Docker instance #558
• Fix bug with ipywidgets and Jupyter #557
• Fix issue with train test split and flat arrays when 2D arrays are expected #552
• Add Graspi notebook and Graspi as an optional dependency #553
• Add a numpy only introduction notebook #548
• Ensure functions and classes work with Numpy not just Dask #546
• Use versioneer for versioning #542
• Add pair correlations functionality #524
• Add conda-forge installation to Circle CI #521
• Fix bug with non-periodic auto-correlations #518