Bug-fix release in the 1.6.x series.

Full changelog: <https://github.com/nipy/nipype/milestone/1.6.1?closed=1

  * FIX: Set DistributedPluginBase.refidx type correctly (https://github.com/nipy/nipype/pull/3340)
  * FIX: change fsl interface randomise --f_only to --fonly for #3322 (https://github.com/nipy/nipype/pull/3325)
  * FIX: BET raising "No image files match: ..." with very long file names (https://github.com/nipy/nipype/pull/3309)
  * FIX: Update SmoothEstimateOutputSpec resels description (https://github.com/nipy/nipype/pull/3316)
  * ENH: Adds interfaces for MRtrix utils shconv and sh2amp (https://github.com/nipy/nipype/pull/3280)
  * ENH: Interface for R (https://github.com/nipy/nipype/pull/3291)
  * ENH: Add CAT12 interfaces (https://github.com/nipy/nipype/pull/3310)
  * ENH: AFNI 3dNetCorr as afni.NetCorr (https://github.com/nipy/nipype/pull/3263)
  * ENH: Skip newline before Python call in batch submission to facilitate containerized runs (https://github.com/nipy/nipype/pull/3297)
  * ENH: Add new dwifslpreproc interface for MRtrix3 (https://github.com/nipy/nipype/pull/3278)
  * REF: Cache nodes in workflow to speed up construction, other optimizations (https://github.com/nipy/nipype/pull/3331)
  * DOC: Fixed Developer Setup Link in install.rst (https://github.com/nipy/nipype/pull/3330)
  * MNT: Blacklist Dipy 1.4.1 (https://github.com/nipy/nipype/pull/3335)
  * MNT: Drop support for numpy < 1.15.3 (https://github.com/nipy/nipype/pull/3284)
  * CI: Build docker images with Python 3.8 (https://github.com/nipy/nipype/pull/3287)
  * CI: Drop Circle doc builds (https://github.com/nipy/nipype/pull/3338)
  * CI: Drop Travis (https://github.com/nipy/nipype/pull/3332)
  * CI: Build docker images with Python 3.8 (https://github.com/nipy/nipype/pull/3287)
  * CI: Add specs and style checks (https://github.com/nipy/nipype/pull/3321)
  * CI: Move from Travis to GitHub actions (https://github.com/nipy/nipype/pull/3318)