Allow geometry material selection from rml

[lobis]

After talking with @cmargalejo offline we thought it might be a good idea to be able to modify the sensitive volume material from an RML (use xenon instead of argon, etc.). This could apply to any volume.

I guess the appropiate place to handle this would be the detector section, we could have something like:

<detector>
    <volume name="gasVolume" sensitive="true" maxStepSize="0.05mm" material="NewMaterial"/>
</detector>

The material would then be replaced in the gdml and all required additions to the <materials/> tag would also be automatically made. The new material could be available in the gdml already or we could use the default materials file.