Nipype-generate-fieldmaps

- FIX: Corrected `output_names` for the `effective_echo_spacing` node so that it matches the trait/parameter linked to the `outputnode`.

- UPDATE: Add `echo_spacing`, `pe1_pedir`, and `pe2_pedir` as workflow outputs
- DOC: Add workflow diagram to README
- DOC: Fix example to use existing output parameters

**Full Changelog**: https://github.com/andrewrosss/nipype-generate-fieldmaps/compare/0.2.2...0.2.3

- FIX: Fixed bug in how CLI is hooked up to the workflow
- UPDATE: Improved documentation (README)
- UPDATE: Added a license (MIT)

- FIX: Prevent acq_params node outputs from being prematurely cleaned-up.
- UPDATE: Rename out_filed output of topup node.
- UPDATE: Add documentation (README)
- UPDATE: Add simple CLI

First release