ENH: Added new fieldExtents function object

Description
    Calculates the spatial minimum and maximum extents of a field

    The extents are derived from the bound box limits after identifying
    the locations where field values exceed the user-supplied threshold
    value.

Usage
    Example of function object specification:

    fieldExtents1
    {
        type        fieldExtents;
        libs        ("libfieldFunctionObjects.so");
        ...
        writeToFile yes;
        log         yes;
        fields      (alpha);
        threshold   0.5;
        patches     ();
    }

    Where the entries comprise:

        Property      | Description              | Required   | Default
        type          | type name: fieldExtents  | yes        |
        writeToFile   | write extents data to file | no       | yes
        log           | write extents data to standard output | no | yes
        internalField | Process the internal field | no       | yes
        threshold     | Field value to identify extents boundary | yes |
        referencePosition | Reference position   | no         | (0 0 0)
        fields        | list of fields to process | yes       |
        patches       | list of patches to process | no       | <all>

    Output data is written to the file \<timeDir\>/fieldExtents.dat

Note
    For non-scalar fields, the magnitude of the field is employed and
    compared to the threshold value.