Calculates and outputs a field whose values are offset to a reference
    value obtained by sampling the field at a user-specified location.

    The field values are calculated using:

    \f[
        f_c = s(f_{c,t} - f_p + f_{off})
    \f]

    where
    \vartable
        f_c     | field values at cell
        s       | optional scale factor (default = 1)
        f_{c,t} | current field values at cell at this time
        f_p     | field value at position
        f_{off} | offset field value (default = 0)
    \endvartable

Usage
    Example of function object specification to calculate the reference
field:
    \verbatim
    pRef
    {
        type        reference;
        libs        ("libfieldFunctionObjects.so");
        ...
        field       p;
        result      pRef;
        position    (0 0 0);
        scale       1.2;
        offset      100000;
    }
    \endverbatim