rename load_file_list_of_signatures in sourmash_args.py

[ctb]

The function load_file_list_of_signatures in src/sourmash/sourmash_args.py was initially used only for loading a text file that contained a list of signature filenames, but since has changed (e.g. #1362) to support lists of FASTA files.

TODO:

• rename the function to load_pathlist_from_file (or a better name if you have one :)
• find all places where the old name is used in the code base, perhaps with git grep -l load_file_list_of_signatures and rename it there
• optionally, add some Python unit tests in tests/test_sourmash.py for this function, including empty file list, badly formatted ones, ones with duplicates, ???