Add missing PUBX config file

Release v4.2.8

Release 4.2.7

Fix SlashDevin#76

Work-around for bad `pgm_read_ptr` in Due core.

Implement SlashDevin#75

*  Add new Velocity North/East/Down members
- values and validity flags added to `gps_fix`
- printed by `trace_all`
- parsed from UBX NAV_VELNED message

Fix SlashDevin#73

*  DIVIDE by scale, not multiply  :-/

v4.2.3

* Examples default to using AltSoftSerial
* Char and field validation implemented for all standard NMEA sentences

Formatting

Rework GPSport.h

* Update Installation page
* Update all examples

Add validation options