Returns the first period without NaNs.

Closes #15

(cherry picked from commit 5da2d18e)

(cherry picked from commit bb2064b6)

Stéphane Adjemian authored Nov 26, 2015 and Stéphane Adjemian committed Feb 18, 2016

... and added a new methods.

Methods with trailing underscore (log_, exp_ and align_) in the name do
inplace modifications, while methods without trailing underscore create
new objects.

Returns the arithmetic or geometric mean and standard deviation.