when creating a bw file from results

* to have at least min num results
* to have results that are more recent that x secs
* to have results that are away from each other x secs

that are away from each other a min num of secs and have at least
x results

useful to combine data when plotting

also:
* fix missing avg bw after moving functions to methods
* add more attrs to V3BWLine that are going to be used by other
  scaling methods

instead of conf and args, so that it's easier to test the method
without creating ConfigParser and ArgumentParser objects.

1. magic methods
2. classmethods
3. staticmethods
4. properties
5. methods

1. magic methods
2. classmethods
3. staticmethods
4. properties
5. methods

1. magic methods
2. classmethods
3. staticmethods
4. properties
5. methods

teor authored Jul 18, 2018 and juga committed Jul 23, 2018

Also:
* document the location of latest.v3bw
* document the atomic creation of latest.v3bw
* explain how to atomically create latest.v3bw when transferring it to
  another host

Closes tor trac 26740.

- Remove started_at file in config et al.
- Add state_fname to config et al.
- Remove read/write functions for the started_at file in scanner.py
  and genearte.py

but not allowing /dev/stdout

This required a bump in the result version: we add a new field to
ResultSuccess.