Improve input file data (time) frame logic

### What is the feature/what would you like to discuss?

Right now the time-step (`data_frame`) used to read `FPHYStest` input data files is simply hardcoded to be the current model time step plus two. However, this is likely not ideal because depending on the file we may not want to skip the first couple of time steps, nor potentially miss out on the last few model time steps.

Given this, it would probably be beneficial to either build in more logic in CAM-SIMA to better determine what the input file time step should be, or to use external snapshot file processing scripts (similar to what is described in issue #233) that automatically remove the first "bad" time step from a snapshot file, thus eliminating the need to skip any file time steps in CAM-SIMA itself.

### Is there anyone in particular you want to be part of this conversation?

@peverwhee @cacraigucar

### Will this change (regression test) answers?

No

### Will you be implementing this enhancement yourself?

Any Software Engineer can do this