*SetField() - single field with user subroutine
Sets the properties of a single field with a user subroutine.
Syntax
*SetField(field_name, USER, 
                                   `solver_expression`) 
*Set(field_name.use_local_dll, TRUE|FALSE) 
*Set(field.local_dll, "string")
    Arguments
- field_name
 - The variable name of an existing field.
 - USER
 - Set to USER for the field to be represented by a user subroutine.
 - solver_expression
 - A solver expression enclosed in back quotes to specify arguments for the user subroutine.
 - field_name.use_local_dll
 - Specifies if a user subroutine DLL is specific to the instance of one coupler entity.
 - field_name.local_dll
 - The path and filename for the user subroutine DLL to be used with an instance of the user defined coupler.
 
Example
*BeginMDL(model, "Field Model")
 *Body(b_0, "Body 0")
 *Body(b_1, "Body 1")
 *Point(p_0, "Point 0")
 *Point(p_1, "Point 1")
 *Point(p_2, "Point 2")
 *Vector(v_1, "Vector 1")
 *Vector(v_2, "Vector 2")
 *Field(fi_0, "Field 0 - Single", b_0, b_1, p_0)
 *SetField(fi_0, USER, `USER(300, 1, 2)`)
 *SetOrientation(fi_0, TWOAXES, XY, v_1, v_2)
 *SetOrientation(fi_0, TWOAXES, XY, p_1, p_2)
 *SetOrientation(fi_0, ONEAXIS, X, p_1)
 *SetOrientation(fi_0, ONEAXIS, X, v_1)
 *Set(fi_0.use_local_dll, TRUE)
 *Set(fi_0.local_dll, "c:/mypath/my_field_dll")
*EndMDL()