v0.4.0

Release of development version 0.4.0 with updates including:

1. New options define how the peak and tail signal are computed when applying the thresholding for images to determine whether to terminate the fitting.
   

2. New AnisotropicGaussianFit class for fitting images with anisotropic diffusion along the major (x,y) axes; the x and y axes can have different diffusion coefficients.
   

3. Additional output file from the CLI to store the measured diffusion coefficients and some other fitting diagnostics.
   

4. Additional output file from the CLI to store the input arguments used for the diffusionfit run.