ENH: Added new function object to calculate the energy spectrum

Description
    Calculates the energy spectrum for a structured IJK mesh

Usage
    Example of function object specification:
    energySpectrum1
    {
        type        energySpectrum;
        libs        ("libfieldFunctionObjects.so");
    }

    Where the entries comprise:
    \table
        Property     | Description               | Required    | Default value
        type         | type name: energySpectrum | yes         |
        log          | write info to standard output | no      | yes
    \endtable

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