MAINT: Relicense from BSD-3-Clause to Apache 2.0

[oesteban]

(see nipreps/fmriprep#2325 for the rationale behind this move)

[codecov]

Codecov Report

Merging #581 (5d667ff) into master (3120884) will decrease coverage by 5.40%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #581      +/-   ##
==========================================
- Coverage   64.69%   59.28%   -5.41%     
==========================================
  Files          43       42       -1     
  Lines        5291     5276      -15     
  Branches      774      770       -4     
==========================================
- Hits         3423     3128     -295     
- Misses       1707     2009     +302     
+ Partials      161      139      -22     

Flag	Coverage Δ
documentation	?
masks	?
reportlettests	?
travis	59.28% <100.00%> (ø)
unittests	?

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Impacted Files	Coverage Δ
niworkflows/__about__.py	100.00% <100.00%> (ø)
niworkflows/func/util.py	21.17% <0.00%> (-58.83%)	⬇️
niworkflows/anat/ants.py	9.70% <0.00%> (-54.44%)	⬇️
niworkflows/anat/freesurfer.py	39.13% <0.00%> (-52.18%)	⬇️
niworkflows/anat/skullstrip.py	30.00% <0.00%> (-50.00%)	⬇️
niworkflows/interfaces/itk.py	26.31% <0.00%> (-12.04%)	⬇️
niworkflows/interfaces/fixes.py	44.44% <0.00%> (-11.12%)	⬇️
niworkflows/interfaces/bids.py	87.62% <0.00%> (-8.70%)	⬇️
niworkflows/interfaces/utils.py	37.76% <0.00%> (-4.99%)	⬇️
niworkflows/interfaces/images.py	66.15% <0.00%> (-3.93%)	⬇️
... and 4 more

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