Use helper functions related to files and DSDs

This refactors a lot of code to use some new helper functions instead of duplicating code. It also includes these changes:

In the XML parsing, namespaces are now stripped from both elements and attributes
Classes that fetch remote files can use a "request_params" parameter to control the behavior of the remote request
Several new helper functions related to DSDs:
- get_indicator_id_from_series_code
- get_indicator_code_from_series_code
- get_indicator_title_from_series_code
- get_series_code_from_indicator_id
- get_series_code_from_indicator_code
- get_series_code_from_indicator_title

open-sdg / sdg-build