td_get: Downloads data of Brazilian government bonds directly from the website
Description
This function looks into the tesouro direto website
(https://www.tesourodireto.com.br/) and
downloads all of the files containing prices and yields of government bonds.
You can use input asset_codes to restrict the downloads to specific bonds
Strings that identify the assets (1 or more assets) in the
names of the excel files. E.g. asset_codes = 'LTN'. When set to NULL, it
will download all available assets
first_year
first year of data (minimum of 2015)
last_year
first year of data
dl_folder
Name of folder to save excel files from tesouro direto (will
create if it does not exists)