ENH: Updated Curle function object

The function object now computes the acoustic pressure at a list of user
specified locations, or from the face centres of a user-supplied surface.
When operating on an input surface, the output can be written back to the
surface or as a list of point values.

Example of function object specification:

    Curle1
    {
        type            Curle;
        libs            ("libfieldFunctionObjects.so");
        ...
        patches         (surface1 surface2);
        c0              330;

        // Input - either points or surface

        input           points;
        observerPositions ((0 0 0)(1 0 0));

        //input           surface;
        //surface         "inputSurface.obj"

        // Output - either points or surface
        output          points;

        //output          surface;
        //surfaceType     ensight;
    }

    Where the entries comprise:
        Property     | Description                  | Required | Default value
        type         | Type name: Curle             | yes      |
        p            | Pressure field name          | no       | p
        patches      | Sound generation patch names | yes      |
        c0           | Reference speed of sound     | yes      |
        input        | Input type                   | yes      |
        observerPositions | List of observer positions (x y z) | no      |
        surface      | Input surface file name      | no       |
        output       | Output type                  | yes      |
        surfaceType  | Output surface type          | no       |