* Check if netCDF output file already exists, and prompt the user
before overwriting.
* Check if input file exists before trying to convert it.
* Some support for series files.
* Some more metadata for identifying horizontal and time dimensions.
* Some support for masked data. Detects records that have an '' in
their typvar, and looks for a corresponding mask record (typvar='').
* New --ignore-etiket option to allow records with different etikets
to be combined into a single output variable.