Set timeseries classes to implement special methods

[wilsonfreitas]

Name the returned timeseries of functions:

• get_series
• get_currency

with an additional class named rbcbTimeSeries, for example.

This class can used to implement methods that:

• return the currency name for data returned from get_currency
• a special merge method to join returned time series

[wilsonfreitas]

The series object must be improved.

SGS series have the following attributes:

• code
• freq
• unit
• description
• start and end
• name
• data

many of these informations can be obtainded thru web scraping, it should be very good if it could be accessed thru an API.

The new object can inherits the base class (dataframe, tibble, or other) and might have an attribute called info.
The print/show method calls the ancestor method.

Smart methods should be created to make convertions in frequency, pex.