Put motor group under 'axis' node in redis and for Flint

For file writer, and HDF5 in particular, scans with multiple top-masters
are handled as individual scans with a '.X' suffix appended to the
scan number, where X represents the subscan number: 1,2,...

By default any top master will stop the scan when its acquisition is
finished. The 'terminator' property can be set to False to indicate
the scan will not be finished when this top master ends.

motor groups have no name, hence each channel (motor in group)
is attached to its parent node to make it look like it
corresponds to its own dataset using the motor name.

A new master node in file is not created for each master in scan.
Removed intra-file references.

Added 'reset_stats()' method to clear the dict (called from the scan)

$ 2to3 -w .
$ 2to3 -w bin/*
$ black bin/*

Check that the same amount of position for motors

- Get reproducible output and reading of class attributes under python2 and python3

The hook class have prepare, start, stop callback methods.

This is usefully if you want to have a motor position for any acquisition
points.

Avoids confusion

* ScanSaving objects can have a name to store data in redis, if no name
is given an unique identifier is generated
* ScanSaving: added 'scan_number' and 'scan_number_format', removed 'run_number'
* ScanSaving: removed 'scan' key replaced by 'scan_name'
* added 'data_path' to dict returned by ScanSaving.get()
* Scan: changed constructor signature, "parent", "writer" has to be
passed through a ScanSaving object, if no object is specified the default
ScanSaving object for the session is used. Added "save" flags.