Loads preweigh data. Called by analysis_pet(). Calls FileFindFirst(), FindHeader(), preweigh_load(), and clean_list(). This function checks the BLR file for preweigh data by searching for the appropriate header. If it is found, it is loaded. Otherwise, preweigh_load() is called, and the user must select a preweigh data file (a WGH file). This data is then loaded, and copied into the BLR file. Blood cannot be analyzed without a preweigh data file, and associated sample set.