Draft DWIDenoise2 interface

[tsalo]

Closes #1070.

In addition to fixing any bugs from the change, we need to do the following:

• Identify appropriate defaults (if they differ from dwidenoise2's defaults) for magnitude-only and complex-valued dMRI data.
• Pass in additional useful informations (e.g., bval and bvec files).
• Support a dwidenoise2 parameter parsing setup like --denoise-method dwidenoise;demodulate:nonlinear (e.g., semicolons separating parameters, colons separating parameters from their values, dropped leading dashes). The exact syntax is up for debate.

Changes proposed in this pull request

Documentation that should be reviewed