Tree marker tau's are not measured from t_peak.  Also added some tree analysis functions.
Added two-way match check to read_matches() and now use this to check emerged halo match validity.  May need debugging still.
Debugging of 2-way match code and emerged halo fix (previous commit).
Numerous important refinements to the treatment of progenitor ordering in gbpTrees.
Significant debugging of gadget_excise and a few other things.
Applied patch from pangel to fix make_gadget_pspec.c