value_compare
Description:
Compares two values of the same type, with the exception that a value of any type can be compared to a GDA_TYPE_NULL value, specifically: <
      itemizedlist>  <listitem><para>if value1 and value2 are both GDA_TYPE_NULL values then the returned 
      value is 0</para></listitem>  <listitem><para>if value1 is a GDA_TYPE_NULL value and value2 is
       of another type then the returned value is -1</para></listitem>  <listitem><para>if value1 is of another 
      type and value2 is a GDA_TYPE_NULL value then the returned value is 1</para></listitem>  <listitem><para>in
       all other cases, value1 and value2 must be of the same type and their values are compared</para></listitem
      > </itemizedlist>
Parameters:
| value1 | 
           a Value to compare (not null)  | 
      
| value2 | 
           the other Value to be compared to   | 
      
Returns:
| 
           if both values have the same type, returns 0 if both contain the same value, an integer less than 0 if   |