Merge branch 'hotfix/v0.3.2'
implemented __pow__ magic method to SpectralPowerDistribution object
implemented __pow__ magic method for TriSpectralPowerDistribution object
fixed typo in spd doctest and fixed errors in tri_spd __pow__ example
Merge pull request #138 from canavandl/feature/__pow__magicmethod

PR: Implement "__pow__" magic method support for "colour.SpectralPowerDistribution" and "colour.TriSpectralPowerDistribution" classes.