mppnccombine needs update to use the input format by default

[ceblanton]

Describe the bug
mppnccombine supports Netcdf-3 classic, Netcdf-3 64-bit offset, and Netcdf-4 input file and output file formats.

The FRE team believed that if you specify the output format (i.e. -64 or -n4), then the tool will use the format. Otherwise, it should use the input format.

Unfortunately, this is not how the tool works. How it works is: If you specify the output format (-64 or -n4), then the tool will use that format (Netcdf-3 64-bit offset and Netcdf-4 classic). But if the output format is not specified, then the default is Netcdf-3 classic.

To Reproduce
Find some test input files and convert them so both Netcdf-4 and Netcdf-3 64-bit offset are available.

Run mppncombine without specifying the output format, and observe the behavior.

In both cases, the tool chooses the never-desired Netcdf-3 classic format.

(use ncdump -k to see the file type)

Expected behavior
When the input files are either Netcdf-3 64-bit offset or regular Netcdf-3, and no output format is specified, then the output format should be Netcdf-3 64-bit offset.

When the input files are Netcdf-4 and no output format is specified, then the output format should be Netcdf-4.

System Environment
any

Additional context
Add any other context about the problem. If applicable, include where any files
that help describe, or reproduce the problem exist.

[ceblanton]

@HansVahlenkamp, as the original and most recent mppnccombine maintainer, could you fix this? Thank you!