- New class member tags

 - Member tags is a structure, initialized to empty.

 - Each user-defined field of this structure must be a vobs(o)*1 cell
   array (each element is associated to a variable).

 - To add a new tag name:

      >> o.tag('type')

 - To associate a tag value to a variable:

      >> o.tag('type', 'Consumption', 'Flow')

   the first argument is the tag name, the second argument is the name
   of the variable, the last argument is the value of the tag.

This member keeps track of the transformations and operations on the
variables. The name of the variables is now constant.

Houtan Bastani authored Apr 06, 2017 and Stéphane Adjemian committed Apr 20, 2017

(cherry picked from commit 8decc584)

GENERAL RULE: Methods with trailing underscore do in place modifications of the
object, while methods without trailing underscore do a copy (and leave
the original object unchanged).

Also fixed some headers and unit tests + cosmetic changes.

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.