Updated GenXSecAnalyzer and GenLumiInfoProduct for combining files correctly

Previous version could only combine files with exactly the same number of LHE processes or Pythia MC.

However, we could have two files that come from the same physics, and contain different number of LHE processes (for example, one contains process 0 and 1 and the other contains process 0, 1, 2).

In order to take this into account properly, there is a minor change in GenLumiInfoProduct, 
and a major change in GenXSecAnalyzer. 

In addition, there is an update regarding the combination of parton-shower MC samples. Previous version computes the combined cross section before GenFilter and multiply the combined cross sections with overall GenFilter efficiency. The updated version obtains the product of GenFilter efficiency and cross section 
first, then, do an weighted average to get the combined cross section.

There is also a dummy change in LHERunInfo.cc (which gives the same result as before).