Using constants in XML files

[rete]

BEGINRELEASENOTES

• Added a constants section in XMLParser parsing :
  • Allows to write constants and refer to in processor parameter values and global parameter values
  • Makes use of ${constantName} to refer to a constant
  • Use command line argument to override a constant
  • example:

  <constants>
    <constant name="MCParticleCollection" value="MCParticle"/>
    <constant name="FilePath" value="../../test/testmarlin"/>
    <constant name="InputFile" value="${FilePath}/simjob.slcio"/>
  </constants>

Marlin --constant.InputFile=aDifferentFile.slcio  marlin.xml

ENDRELEASENOTES

This has been tested with the test processor in the examples/mymarlin directory as for #19 .
Test it with this file : marlin.txt (rename it with .xml extension)

# from Marlin root dir :
cd examples/mymarlin/
mkdir build
cd build
cmake -C $ILCSOFT/ILCSoft.cmake .. && make install
cd ..
Marlin marlin.xml

[gaede]

Remi, can you add command line overwrite for the constants to this PR as well ?
Syntax should be the same as for the parameters: --constant.constanName=Value.

[rete]

@gaede : Should have I to add also the feature with environment variables we talked about ?
If not, I'm done with this PR

[rete]

I also added a cmake test to test this feature, plus the recent include mechanism from #19