Transmission window implementation

[jgalan]

This process will allow to define a combination of windows that will be used to calculate the transmission.

Inside the process one defines the name of the TRestAxionXrayWindow that wants to be implemented. The process will combine them to create a unique window definition with different layers.

For example:

        <addProcess type="TRestAxionTransmissionProcess" name="window" verboseLevel="debug" value="ON">
            <parameter name="center" value="(0,0,7500)mm" />
            <observable name="transmission"/>
            <window name="cathode" />
            <window name="strongBack" />
        </addProcess>

The TRestAxionXrayWindow should be defined inside TRestRun.

<TRestRun ...>
...
        <TRestAxionXrayWindow name="strongBack" verboseLevel="warning">
            <parameter name="center" value="(0,0,0)mm" />
            <parameter name="thickness" value="200um" />
            <parameter name="material" value="Si" />

            <TRestGridMask name="mask" verboseLevel="warning" maskRadius="8mm" offset="(2,2)mm" 
                rotationAngle="0.5" gridGap="4mm" gridThickness="0.5mm" />
        </TRestAxionXrayWindow>

        <TRestAxionXrayWindow name="cathode" verboseLevel="info" center="(0,0,0)mm" thickness="0.02um" material="Al" />
....
</TRestRun>

[jovoy]

A value that combines all probabilities like window_transmission and optics_efficiency is still missing but that does not necessarily belong in this PR.

[jgalan]

A value that combines all probabilities like window_transmission and optics_efficiency is still missing but that does not necessarily belong in this PR.

This was discussed somewhere else. Efficiencies can be combined later on using the generated ROOT analysis tree