Improve week roll over in tracking

[ljbade]

Clean up the code at

piksi_firmware/src/track.c

Lines 310 to 316 in de6b0a6

	/* Have a valid time of week - increment it. */
	chan->TOW_ms += chan->short_cycle ? 1 : (chan->int_ms-1);
	if (chan->TOW_ms >= 7*24*60*60*1000)
	chan->TOW_ms -= 7*24*60*60*1000;
	/* TODO: maybe keep track of week number in channel state, or
	derive it from system time */
	}

This should be made a function in libswiftnav time.c and use WEEK_SECS constant.

@henryhallam Your thoughts? Also can you explain your TODO?