Changes
- Fix bug in `n_steps` keyword argument.
- Set default number of steps to `100`, based on the observation that (within reason) there was little dependence of the variance of the diffusion coefficient on the number of steps.
- Enable minimum and maximum `dt` values in the parser.
- Enable the use of logarithmic steps in `dt`.
- Some minor refactoring.
- Improvements to estimation of size of `disp_3d` object.
- Improvements to documentation.