add resample as nifti method

Add a method that can resample spatial resolution as well as frames. Both of these functions are already available in the to_niff function, but a method would be nice.

Search the repository for the existing to_niff function and inspect how it handles spatial-resolution and frame resampling. Add a method exposing both capabilities, then verify that the method produces the expected resampled NIfTI output for each case using the project's available tests or examples.

Written by the indexing model from the issue text.