Allow gpt to use template variables in parameter files

It should be possible to use 'gpt' command line arguments as template variables in gpt's parameter files.

For example, it should be possible to use the command-line call:
    gpt Binning -p param.xml -t CHL_10D.nc -PsuperSam=5 *.N1


And have a param.xml file that references the value of command-line arguments "-t CHL_10D.nc" and " -PsuperSam=5", e.g. as follows:

<parameters>
    <startDate>2002-01-01</startDate>
    <endDate>2002-01-10</endDate>
    <binningConfig>
        <numRows>180</numRows>
        <superSampling>${superSam}</superSampling>
        <maskExpr>true</maskExpr>
        <aggregators>
            <aggregator>
                <type>AVG</type>
                <varName>chl</varName>
                <weightCoeff>1.0</weightCoeff>
                <fillValue>NaN</fillValue>
            </aggregator>
        </aggregators>
    </binningConfig>
    <formatterConfig>
        <outputType>Product</outputType>
        <outputFile>${gpt.targetFile}</outputFile>
    </formatterConfig>
</parameters>