to make it less confussing.
Note that stem call it ed25519_master_key instead of
master_key_ed25519, but dir-spec.txt uses master-key-ed25519, which
is the term chosen here.

    * Move V3BWLine to v3bwfile.py
    * Add methods to V3BWLine similar to V3BwHeader
    * Add methods to V3BWLine to implement most of the logic in the class
    * Move functions related to data lines to v3bwfile.py
    * Add functions to generate types of erros
    * Add tests for the class and conftest to read results from file
    * Fix other tests depending on bw lines