Adds support for auto-reporting DOP values

[dotMorten]

Following the example of auto updating HPPOSLLS, here's support for auto-updating DOP. Also related to #127

However I can imagine this starts to get unwieldy if too many messages gets added.

An alternative could be to subscribe to any generic message, and a method to check if there's a new message of that type received.
Something like :

bool enableAutoUpdate(class, id, buffer, length);
bool disableAutoUpdate(class, id);
bool getAutoUpdate(class, id);

And use it something along the lines of:

uint8_t dopBuffer[18]
void setup() {
  bool isOk = enableAutoUpdate(UBX_CLASS_NAV, UBX_NAV_DOP, &dopBuffer, 18);
}
void loop()
{
   if(getAutoUpdate(UBX_CLASS_NAV, UBX_NAV_DOP)) // True if buffer was updated since last call to this
   {
      // process dopBuffer;
     uint16_t verticalDOP = gps->extractInt(dopBuffer, 10);
   }
}

Under the covers the existing autoPVT and autoHPPOSLLS could just use these methods.

[PaulZC]

Hi Morten (@dotMorten ),

Can you please target this PR at the release_candidate branch, instead of master? Then I can merge and test it without changing the master branch. Same goes for #142 please.

I need to update CONTRIBUTING.md. It should look like this:
https://github.com/sparkfun/OpenLog_Artemis/blob/master/CONTRIBUTING.md

Many thanks!
Paul

[PaulZC]

Many thanks Morten (@dotMorten),
Sorry, I missed the notification! Merging...
Best wishes,
Paul

[PaulZC]

Hi Morten (@dotMorten),
Thanks again for contributing to the library. Can we please contact you via email? We are working on Version 2.0 of the u-blox library and we would appreciate your thoughts and comments. You can find my email address on my home page:
https://github.com/PaulZC
Very best wishes,
Paul (& Nathan @nseidle)