added internal tempering in grem with example

examples/USER/misc/grem/lj-temper/in.gREM-temper

examples/USER/misc/grem/lj-temper/lj.data

examples/USER/misc/grem/lj-temper/log.lammps.0

examples/USER/misc/grem/lj-temper/log.lammps.1

examples/USER/misc/grem/lj-temper/log.lammps.2

examples/USER/misc/grem/lj-temper/log.lammps.3

examples/USER/misc/grem/lj-temper/run.sh

examples/USER/misc/grem/lj-temper/screen.0

examples/USER/misc/grem/lj-temper/screen.1

examples/USER/misc/grem/lj-temper/screen.2

examples/USER/misc/grem/lj-temper/screen.3

src/USER-MISC/fix_grem.h

src/USER-MISC/temper_grem.cpp

src/USER-MISC/temper_grem.h