Fix GPS Week Number to consider mod 1024

[Duncan-McD]

The GPS Week number for PAN was the wrong value and stuck at 2000. This was determined to be due to the fact that the GPS week number is calculated to be the week number modulus 1024. This value ended up being below the lower serializer bound for the GPS week, causing the value to instead be set to 2000.

More Information from diagnosis by Kyle/Stewart: "We've root caused the GPS time issue where the week number is continually reporting as 2000. Turns out, GPS devices always report the current week number mod 1024. What that means is the Piksi is actually reporting the week number as 144 and this is underflowing the week number serializer. Futhermore, this means the concept of "PAN time" onboard the spacecraft is all out of wack because we calculate PAN time by subtracting another GPS time (the PAN epoch) from with a week number greater that 1024 -- see TimeEstimator.cpp."

More information from old PAN issue ticket: pathfinder-for-autonomous-navigation#864