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LOTS of improvements on various aspects:
* Unified LensingOperator, that supports 'legacy' implementation of nearest-neighbors interpolation, and faster implementations of nearest-neighbors and bilinear interpolations (credits to austinpeel for fast versions)
* Improved adaptive initialisation and refinement of regularisation strength
* Improved noise propagation to source plane and starlet space, with gaussian filtering for better estimation of unconstrained source pixels
* Basic support of image plane supersampling (only nearest-neighbors interpolation for upsampling)
* User can now provide a mask to select pixel that belongs only to lens light, for improving masking of source plane region. Use it with care!
* Better integration with lenstronomy classes