Need better default behavior when setting start_time

[c-white]

If a user specifies the time/start_time input parameter, then the the initial output*/next_time parameters still default to 0 if omitted. Because these are only incremented by dt, next_time will always be start_time less than the current time, so output will be written every timestep.

Now a user can always set all the next_time parameters manually, but I can't imaging ever wanting the default to be 0 when start_time is not 0. I think changing the linked line will fix this, but I wanted to double check.

[jmstone]

This behavior was a problem in Athena4.2. I think the fix used there is that restarts save the output*/next_time data, so when you restart you don't get an output every time step (check Athena4.2 to be sure). Certainly something like this should be implemented in Athena++.

[msbc]

I've added a "restart_tweak" branch to try fixing this issue along with #62. I think (needs testing) I fixed #36 with this line in the outputs constructor, making next_time default to the mesh time if unset.