Enable HLT1-based filtering in HLT2

It's useful for HLT1 line developers to be able to run HLT2 selections which somehow filter on the information produced in HLT1. For example, filtering on the HLT1 decision, or TIS/TOS'ing. We don't currently support this.

For a Moore-based HLT1, we can write out a ROOT file or an MDF. Neither of these will work:

• MDF: the HLT2 selections can only take the reconstruction from the file, it cannot be run on the fly (#160 (closed)). An MDF file only contains the raw event, so we cannot load the reconstruction from an input MDF.
• ROOT: the Moore ROOT output writer is configured to only save the raw event. So the HLT1 application does not propagate the reconstruction already present in the input to the output (#143 (closed)).

Similar statements hold for an Allen-based HLT1.