DVARS calculations for mean null distributions

[FrancescoNottingham]

Added/changed the following functions:

• test_voxel_differences: calculates a 4D volume with different between voxels at t+1 and t
• voxel_iqr_variance: calculates the iqr for each voxel timeseries from above
• distribution_mean: uses the voxel's iqr to calculate the mean for the null distribution

Also added/changed test functions for each of the above

Still to be added:

• Initial normalisation of the data
• calculating the variance for the null distribution

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.51%. Comparing base (3c4256e) to head (bc01e09).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
+ Coverage   90.43%   92.51%   +2.08%     
==========================================
  Files          10       10              
  Lines         115      147      +32     
==========================================
+ Hits          104      136      +32     
  Misses         11       11              

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