NOTE: typical users will never need to use this function.
NetCDF files can be in "define mode", at which time dimensions and variables can be
defined, or new attributes added to a file, or in "data mode", at which time data
can be read from the file.
This call puts a file that is currently in define mode back into data mode.
The ncdf4 package manages this process transparently, so normally, an end user
will not need to call this explicitly.