[FIX] less strict beta map generation

[jdkent]

Summary

Fixes #261

List of changes proposed in this PR (pull-request)

• remove minimum length check in nistats interface
• add length check if correlations are being generated
• test the length check function

[codecov[bot]]

Codecov Report

Merging #262 into master will increase coverage by 0.83%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
+ Coverage   87.61%   88.44%   +0.83%     
==========================================
  Files          10       10              
  Lines         662      675      +13     
  Branches      106      108       +2     
==========================================
+ Hits          580      597      +17     
+ Misses         48       46       -2     
+ Partials       34       32       -2