ENH dwidenoise2: b=0-based noise level estimation

[Lestropie]

From: https://direct.mit.edu/imag/article/doi/10.1162/imag_a_00049/118324/Efficient-PCA-denoising-of-spatially-correlated

This would involve:

• Prior to denoising, extract b=0 volumes, compute variance, median filter
• Store this in a scratch image as an a priori input noise level estimate
• Perform denoising where the "estimator" just reads from that scratch image