No graceful exit when `data_step` is missing from forcing files

[kasra-keshavarz]

Hello,

I just thought to report this. Whenever the data_step is missing from the forcing files, the number of time-steps is reported as follows, as an example:

startTime: iyyy, im, id, ih, imin = 1980  1  1 13  0
finshTime: iyyy, im, id, ih, imin = 1980  1  5 10  0
number of time steps =  **********

I'm not sure if this is a big deal for the model or not, but if it is, probably worth putting an error describing the issue.