Function1 is an abstract base-class of run-time selectable unary
functions which may be composed of other Function1's allowing the user
to specify complex functions of a single scalar variable, e.g. time.
The implementations need not be a simple or continuous functions;
interpolated tables and polynomials are also supported.  In fact form of
mapping between a single scalar input and a single primitive type output
is supportable.

The primary application of Function1 is in time-varying boundary
conditions, it also used for other functions of time, e.g. injected mass
is spray simulations but is not limited to functions of time.

e.g.

    inlet
    {
        type                flowRateInletVelocity;
        massFlowRate        0.2;
        extrapolateProfile  yes;
        rho                 rho;
        rhoInlet            1.0;
        value               uniform (0 0 0);
    }

This reverts commit c5bea524.

This reverts commit b18f6cc1.

- improve documentation, use GPL version 3

- improve documentation, use GPL version 3