cicwi/PyCorrectedEmissionCT: 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: https://github.com/scikit-image/scikit-image/pull/648