Enhancement of the CommonViewPeriod

[vicalloy]

1. CommonViewPeriod::first_track_offset_nanos() returns zero if it is not a bipm_common_view_period.

The 4-minute offset is only applicable to BIPM; for custom periods, 0 might be more reasonable.

# CGGTTS-Version 2E  2.1
These tracking epochs are offset by 4 minutes each day; this offset was originally
fixed in order to stay aligned with the sidereal period of the GPS constellation and to follow each
day the same common-view comparison schedule. Even if the daily repeatability is not needed
any more and is not valid for the constellations other than GPS, it has been decided to keep the
same tracking schedule in order to assure the full compatibility with the previous version of the
CGGTTS.

2. CommonViewPeriod::next_period_start() adds a return value to indicate whether the next CommonViewPeriod is t0.

After calling CommonViewPeriod::next_period_start(), we should know if it's t0. Without this information, we won't know how to preserve the measurement data during the overlap period.
If it's t0, we should start recording the measurement count in the tracker and do nothing else. On the next t (t_89), we remove the recorded count from the tracker.
NOTE: This logic only works when there is an overlap period spanning days(BIPM period). If there is no overlap period, the tracker should always be reset before starting a new tracking.

                        │                                         │       
                        ├──────────────► T0     ◄─────────────────┼       
                        │                                         │       
                                                                          
                   MJD 59026(T0)     MJD 59025(T89)                       
  │                     │                 │                       │       
 ─┼─────────────────────┼─────────────────┼───────────────────────┼───────
  │                     │                 │                       │       
23:50                00:02              00:03                  00:18      
                                                                          
  │                                       │                               
  ├──────────────►  T89     ◄─────────────┤                               
  │                                       │