WIP: Revision of dMRI artifact correction workflows

A lot of revisions under this PR. It'd be interesting to have it included into the next release.

* Moved workflows from `nipype.workflows.dmri.fsl` to `nipype.workflows.dmri.preprocess` for two reasons: 1) unlike Interfaces, workflows are not oriented to tools and therefore they might benefit from using several tools; 2) preprocess is a better name, as I intend to include resampling and denoising as in (http://nipy.org/dipy/examples_built/reslice_datasets.html#example-reslice-datasets, http://nipy.org/dipy/examples_built/denoise_nlmeans.html#example-denoise-nlmeans)
* New names convention to make it easier to understand
* Very much improved documentation
* A lot of fixes in all the pipelines
* There will be a combined pipeline with just one interpolation as suggested here: https://github.com/nipy/nipype/pull/530#issuecomment-14505042