Fix TOW for I/NAV word type 16

[odrisci]

This should fix #162

Tested on a local build of ubxtool.

The concept:

• Word Type 16 should occur either at subframe index 7 or index 14
• Log the most recently decoded satellite TOW (from WT 0, 5 or 6)
• If the most recent TOW < 15 seconds into the current 'cycle' then assume we are at subframe index 7
• Otherwise we are at subframe index 14