* 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

also:
* fix again number of header lines
* FIXME: tests for bw lines should be independent on headers,
  Otherwise they'll fail everytime we change headers.

Also:
* add latest_bandwidth header
* add file_created header
* fix bw files header tests
* disable tests that assume timestamp to be the file creation
time
* if we use ISO 8601 with T as separator,
  datetime format string is not needed

- Create constants for the separators
- Create methods to generate the header text
- Create method to parse the header text
- Fix tests

This reverts commit 628d21a9.

This reverts commit c8b47bb5.

Can be overridden by --output option to sbws generate

GH: closes #91

GH: closes #97

May have some runtime bugs still. Needs a little more testing

GH: closes #76

GH: ref #69

GH: ref #69