DOC: Function docs and CHANGES updates for external imports

Author: mwaskom

CHANGES

@@ -9,6 +9,9 @@ Next release
 * ENH: Minor improvements to FSL's FUGUE and FLIRT interfaces
 * ENH: Added optional dilation of parcels in cmtk.Parcellate 
 * ENH: Interpolation mode added to afni.Resample
+* ENH: Function interface can accept a list of strings containing import statements
+       that allow external functions to run without their imports defined in the
+       function body
 
 * FIX: SpecifyModel works with 3D files correctly now.
 

doc/users/function_interface.rst

@@ -71,6 +71,11 @@ be imported within the function itself::
 Without explicitly importing Nibabel in the body of the function, this
 would fail.
 
+Alternatively, it is possible to provide a list of strings corresponding
+to the imports needed to execute a function as a parameter of the `Function`
+constructor. This allows for the use of external functions that do not
+import all external definitions inside the function body.
+
 Hello World - Function interface in a workflow
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