OPT: Pre-filter dense field transform to speed up repeated map calls

[chriscline]

Addressing issue #288

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.68%. Comparing base (7845e64) to head (61c6986).

Files with missing lines	Patch %	Lines
nitransforms/nonlinear.py	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
- Coverage   96.74%   96.68%   -0.06%     
==========================================
  Files          16       16              
  Lines        1994     1992       -2     
  Branches      267      226      -41     
==========================================
- Hits         1929     1926       -3     
  Misses         41       41              
- Partials       24       25       +1     

Flag	Coverage Δ
unittests	96.68% <90.00%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.