Consider adding unit tests (and in particular around the GPS/NMEA parsing)

Hi,

There are no unit tests for the library; it would be great if there were some so that future library changes can be tested before being deployed. Imagine someone updating their NuGet packages and finding their sensor no longer works as they expected!

For example, some unit tests around parsing of the date time would be ideal, as currently there seems to be a workaround:

https://github.com/adafruit/AdafruitClassLibrary/blob/91d444ccb6a6934ef352c1eed255f771c9e19ba6/AdafruitClassLibrary/GPS.cs#L904-L916

Another area that definitely needs unit testing is the parser functionality. There are several methods like the snippet below. What happens if, for example, the GPS module stops communicating because it crashed? If this happened during a serial transmission of the NMEA string you are **very** likely to have an incomplete string and you will encounter a `IndexOutOfRangeException` because the code is looking at specific elements. As far as I can tell, this exception will then bubble up for the user to deal with. In combination with unit testing, a robust parser can be built which can return a "sorry, this is a bad string, can't do anything with it" result instead of leaving it to chance and forcing the user to deal with out of bounds exceptions.

```
public GPSRMC(string sentence)
{
string[] splitString = sentence.Split(new char[] { ',', '*' });

TimeStamp = ParseTimeStamp(splitString[0]); // Time Stamp
Valid = splitString[1] == "A" ? true : false; // validity -A - ok, V - invalid
Latitude = ParseDouble(splitString[2]); // current Latitude
LatHemisphere = splitString[3]; // North/ South
Longitude = ParseDouble(splitString[4]); // current Longitude
LonHemisphere = splitString[5]; // East/ West
Speed = ParseDouble(splitString[6]); // Speed in knots
Course = ParseDouble(splitString[7]); // True course
DateStamp = ParseDateStamp(splitString[8]); // Date Stamp
MagVariation = ParseDouble(splitString[9]); // Magnetic Variation
VarDirection = splitString[10]; // East/ West

LatDegrees = ToDegrees(Latitude);
LonDegrees = ToDegrees(Longitude);
}
```

Start with GPS.cs, especially the linked date-time parsing workaround around lines 904-916 and the GPSRMC constructor shown in the issue. Add focused tests for date parsing and incomplete NMEA sentences, with completion demonstrated by coverage of malformed input without an IndexOutOfRangeException.

Written by the indexing model from the issue text.