Fix stacking of results

[mimischi]

Fixes #58

Changes made in this Pull Request:

• Changed np.hstack to np.vstack in _conclude() method in pmda/custom.py.

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[codecov]

Codecov Report

Merging #64 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #64   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files           7        7           
  Lines         274      274           
  Branches       26       26           
=======================================
  Hits          272      272           
  Misses          1        1           
  Partials        1        1

Impacted Files	Coverage Δ
pmda/custom.py	100% <100%> (ø)	⬆️

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