clear_fix() patch - for when you put the GPS module to sleep

I've got a project where I put the GPS module to sleep to preserve battery, and then I put the ATMega chip to sleep as well. When it wakes up, get_position() thinks that the fix age has not dramatically increased, because the clock has been stopped while everything was asleep. So I get stale data for a few seconds before the fix age gets long enough that I "notice."

clear_fix() blows away the internal state as if you had created a whole new TinyGPS object.