Add trip distance. Simplify parseCoords(). parse RMC only if valid fix.

[SixFingersMike]

• Scope of change--i.e. what the change does and what parts
  of the code were modified.**

-- Changes to RMC processing

Add trip distance functionality
A new private variable 'driftTolerance' added initialized to 10.0 (meters)
A new public variable 'tripDistance' to record cumulative distance
A new pair of location variables 'latitudeDegreesLocked' and 'longitudeDegreesLocked'

Distance 'd' is calculated between current fix and 'latitudeDegreesLocked' /
'longitudeDegreesLocked'. If 'd' is greater than 'driftTolerance' then 'latitudeDegreesLocked'
and 'longitudeDegreesLocked' are updated to reflect the current fix and tripDistance is
increased by 'd'. This is to filter out small changes in fix postion while actually stationary
or moving slowly.

RMC parsing only takes place if fix is valid i.e. 'A'. To do otherwise does not make sense as
there will be no data in the RMC sentance or would be unreliable anyway.

'driftTolerance' can be set by calling ::setDriftTolerance(x) where x is specified in meters as a float.
'tripDistance' can be reset to zero (or set to any other value)

-- Simplify parseCoords()
Removed several steps and variables

No limitations introduced

Code fully tested