major changes in surface_physics.f90 and example (Makefile, example.namelist, example.f90)

surface_physics:
  * account for prognostic snow albedo 'alb_snow'.
  * added new model parameters as namelist parameters.
example.f90:
  * added support for replacing snow height 'hsnow'
plot_example.py:
  * now also plots 1-1 with correlation coeffcient
Makefile:
  * 'make run_example' automatically compiles, runs, and calls the plot scripts.