Adds write_trajectory in xdr.f90
Adds trjconv2com.f90 to convert a trajectory file to COM trajectory

which takes much less space
Adds COM mode to decompose_mpi to incorporate trjconv2com ...

also fixes some error messages in 'ifort -check all' mode
Adds symlink to trjconv2com in bin
Fixes a bug in trjconv2com that writes the wrong COM coords
Fixes potential bugs of initializing arguments in decompose_mpi.f90
Merge branch 'trjconv2com'
Updates Makefile.pgi and Makefile.ifort