Hotfix: Import NO_STRETCH_SG_PARAMS in compare_*py plot modules

gcpy/plot/compare_single_level.py
gcpy/plot/compare_zonal_mean.py
- Import NO_STRETCH_SG_PARAMS from gcpy.constants.  This had
  been omitted in PR #374.

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>

Author: yantosca

gcpy/plot/compare_single_level.py

@@ -24,7 +24,7 @@
     all_zero_or_nan, slice_by_lev_and_time, compare_varnames, \
     read_config_file, verify_variable_type
 from gcpy.units import check_units, data_unit_is_mol_per_mol
-from gcpy.constants import MW_AIR_g
+from gcpy.constants import MW_AIR_g, NO_STRETCH_SG_PARAMS
 from gcpy.plot.core import gcpy_style, six_panel_subplot_names, \
     _warning_format, WhGrYlRd
 from gcpy.plot.six_plot import six_plot

gcpy/plot/compare_zonal_mean.py

@@ -24,7 +24,7 @@
     all_zero_or_nan, compare_varnames, \
     read_config_file, verify_variable_type
 from gcpy.units import check_units, data_unit_is_mol_per_mol
-from gcpy.constants import MW_AIR_g
+from gcpy.constants import MW_AIR_g, NO_STRETCH_SG_PARAMS
 from gcpy.plot.core import gcpy_style, six_panel_subplot_names, \
     _warning_format, WhGrYlRd
 from gcpy.plot.six_plot import six_plot