#361 Use Netcdf-3 64-bit offset format for split-ncvars single-file …

[ceblanton]

Description
split-ncvars's "single file output" option must use Netcdf-3 64-bit offset format. Bronx frepp exclusively uses the -f single-file option to create a static pp file containing variables from different history files. frepp scripts run split_ncvars.pl f looping over the source history files. With Netcdf-4, this usage reliably causes this error:

ERROR NC_ELATEFILL (formerly NC_EFILLVALUE) Attempt to define fill value when data already exists
HINT: NC_ELATEFILL errors can occur when NCO attempts to create, modify, or overwrite a _FillValue attribute
for an existing variable in a netCDF4 file. The netCDF4 format (unlike netCDF3) does not permit this.

Fixes #361

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Checklist:

• My code follows the style guidelines of this project
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• New check tests, if applicable, are included
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