This PR:

• Makes get_stac_data() more modular, improving the flow and readability of this function;
• Adds download_function as an argument to get_stac_data(), which fixes Make the download method in get_stac_data() a controllable argument? #28
• Dramatically simplifies the simple/complex forks of rsi_download_rasters(), providing a unified download interface instead
• Renames default_query_function() to rsi_query_api()
• Fixes get_stac_data() should check for assets existing at the top level #53
• Calculates progress bars separately for each stage of downloading, rather than using a single bar for the entire function