TRZ reader default dt value differs from base default value

[IAlibay]

Expected behavior

The default value for dt is the same across all readers.

Actual behavior

As pointed out in #3256 the TRZ reader defaults to 0 instead of the base value of 1.0. I propose in 2.1.0 that we just change it to align the behaviour with the other readers (as per the warning I added in #3256).

[richardjgowers]

The 0 value only happens when there's only one frame so this is very minor. But yeah we can fix this once 2.0 is out the door.

[IAlibay]

I'm fixing some TRZ behaviour somewhere else [for 2.0.0], I might just throw the warning in there so we can make 2.1.0 the "get rid of all the warnings" release.