Fix further bug in Tube returning wrong time
Fix further bug in Tube returning wrong time

-- This bug returned 0 too soon. It was looking at the solutions for times to one of the Z boundaries and if neither of the roots of the equation were suitable it was returning 0 without checking the time to the other boundary.  
mjrasobarnett/ucnsim,mjrasobarnett/ucnsim,mjrasobarnett/ucnsim