fix stream full_file mode

[peterdschwartz]

The "full_file" read mode for streams appears to have stopped being supported at some point. After looking at all the stream settings, I could not figure out a way to enable shr_dmodel_readstrm_fullfile to work correctly, so I attempted to re-write the subroutine so that it essentially works like shr_dmodel_readstrm but with a loop over all the time steps present in the file, using pio_setframe to go through the time slices.

If someone knows of how to get the original function to work or a better way to implement this, I would love to use that instead.

Fixes #6742

[peterdschwartz]

This branch needs more testing so keeping it as a draft. But I would like feedback as I do not have much experience modifying the streams or with the inner workings of pio.

@jayeshkrishna @rljacob

[peterdschwartz]

I ran the elm test suites and some ERS tests fail. I assume this is because there needs to be some additional logic about which timeslice to copy from the avFile buffer.
@jayeshkrishna Have you had a chance to look at this yet?

[jayeshkrishna]

@peterdschwartz : Sorry about the delay, I will get to this change this week