Update the code to search for a sensible index for the position arrays.

It now works under the following conditions:

1) If no variables have been specified when calling the function.
2) If only oddly gridded data have been specified
3) If a combination of oddly and normally gridded data have been specified
4) If only normally gridded data have been specified

Hopefully that makes it relatively robust.
Update the code to write out pressure data if using either 'slp' or 'pres' as the variable name. Hopefully if the data have been obtained via get_NCEP_forcing, then it will always be 'pres', but better safe than sorry