FIX: Import NUMPY_MMAP inside functions

[effigies]

Follow-up to #1796. Some imports were at the file level when they should have been at the function level. My criterion for moving was whether there are other function-local imports.

Addresses issue discovered in nipreps/fmriprep#362 (and nipreps/fmriprep#363).

[effigies]

cc @oesteban @satra for review. (Apparently requesting review is a privileged act? Weird.)

[oesteban]

Good catch

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@102d8d7). Click here to learn what that means.
The diff coverage is 0%.

@@           Coverage Diff            @@
##             master   #1831   +/-   ##
========================================
  Coverage          ?   72.6%           
========================================
  Files             ?    1054           
  Lines             ?   52522           
  Branches          ?       0           
========================================
  Hits              ?   38134           
  Misses            ?   14388           
  Partials          ?       0

Flag	Coverage Δ
#unittests	72.6% <ø> (?)

Impacted Files	Coverage Δ
nipype/workflows/dmri/dipy/denoise.py	20.89% <ø> (ø)
nipype/workflows/dmri/fsl/utils.py	19.69% <ø> (ø)
nipype/workflows/dmri/fsl/epi.py	46.51% <ø> (ø)
nipype/interfaces/dipy/preprocess.py	23.25% <ø> (ø)
nipype/workflows/dmri/fsl/tbss.py	54.2% <ø> (ø)
nipype/workflows/misc/utils.py	15% <ø> (ø)

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