Typically, users will never need 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 data mode back into define mode.
This functionality is handled transparently by the ncdf4 library, so users
will never need to call this unless they are doing advanced manipulations
of netcdf files.