Release version 0.6.0

New major release, adding the following features and fixing the following bugs:

Added

• Multi-channel (collaborative) regularization (including TNV - Total Nuclear Variation).
• Support for custom (external, user defined) projection backends.
• Dedicated FBP filters module.
• Parallelization of guided regularization functions.
• A lot of work towards full type hinting usage.

Fixed

• Potential memory leak in astra-toolbox projector backend.
• Gradient and laplacian implementations for arbitrary directions.
• Output and consistency from tests for solver classes.
• Residual norm computation in the weighted least squares data term.
• Dimension mismatch in projector, when using only one angle.
• Back-projection normalization in scikit-image projector backend.

The scikit-image and astra-toolbox backend still present an inconsistent rotation center position definition. There is now a PR to fix the rotation center position in scikit-image: scikit-image/scikit-image#6489