ENH: Added new AMIWeights function object

Reports the min|max|average AMI weights to text file and optionally
writes VTK surfaces of the sum of the weights, and mask field for
ACMI patches.

Example usage:

    AMIWeights
    {
        type            AMIWeights;
        libs            ("libfieldFunctionObjects.so");
        writeControl    writeTime;
        writeFields     yes;
    }