store "num_io_buffers" in memory to reduce memory allocations. 
   distributed reads are supported using the optional "domain" flag.  
   Units conversion via the optional "desired_units" flag using udunits_mod.
   <br>
<br>
   Return integer id of field for future calls to time_interp_external.
   <br>
<br> 
</dd>
<br>
<br>
<dt>
<b>INPUT</b>
</dt>
<dd>
<table border="0">
<tr>
<td valign="top" align="left"><tt>file&nbsp;&nbsp;&nbsp;</tt></td><td>   filename <br>&nbsp;&nbsp;&nbsp;<span class="type">[characte