PERF: Avoid exp and log (about 4x faster for integer scalar)
PERF: Avoid exp and log (about 4x faster for integer scalar)

Performance is similar for floating point scalar
b/c it needs to end up computing the same thing:
In [29]: %timeit np.power(a, 2)
10 loops, best of 3: 22.5 ms per loop

In [30]: %timeit np.exp(np.log(a) * 2)
10 loops, best of 3: 96.2 ms per loop

In [31]: %timeit np.exp(np.log(a) * .2)
10 loops, best of 3: 96.2 ms per loop

In [32]: %timeit np.power(a, .2)
10 loops, best of 3: 100 ms per loop

Achuth17/scikit-bio,johnchase/scikit-bio,SamStudio8/scikit-bio,jairideout/scikit-bio,colinbrislawn/scikit-bio,xguse/scikit-bio,anderspitman/scikit-bio,corburn/scikit-bio,kdmurray91/scikit-bio,Achuth17/scikit-bio,demis001/scikit-bio,colinbrislawn/scikit-bio,corburn/scikit-bio,wdwvt1/scikit-bio,jdrudolph/scikit-bio,demis001/scikit-bio,averagehat/scikit-bio,jensreeder/scikit-bio,anderspitman/scikit-bio,jairideout/scikit-bio,gregcaporaso/scikit-bio,johnchase/scikit-bio,jdrudolph/scikit-bio,jensreeder/scikit-bio,gregcaporaso/scikit-bio,averagehat/scikit-bio,SamStudio8/scikit-bio,wdwvt1/scikit-bio,kdmurray91/scikit-bio,xguse/scikit-bio