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

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.

When NaNs are present in the dataset. Do not return an array filled with
NaNs if only the first observations are NaNs.

Do not test the equality of dseries object but the equality of data
members instead.

Third test was only testing that a dseries object is equal to itself.