Bin no_trees_on_this_level, use exp. decay away from snow line.

Probability of placing an arctic tree is now an exponential decay
function of height distance from the snow line, instead of the previous
50% blocks.
This results in a more gradual thinning out of arctic trees in
each direction.
The algorithm is: p = exp(-3 * distance / range_setting),
using a rather crude approximation of the exponential function.

The no_trees_on_this_level setting is not really useful, and its
dual behaviour to disallow discrete height levels and turn on the
snow line behaviour is unintuitive.
Replace it with a simple on/off setting.

src/table/settings.ini

@@ -2310,23 +2310,22 @@ strhelp  = STR_CONFIG_SETTING_EXTRA_TREE_PLACEMENT_HELPTEXT
 strval   = STR_CONFIG_SETTING_EXTRA_TREE_PLACEMENT_NONE
 cat      = SC_BASIC
 
+[SDT_BOOL]
+base     = GameSettings
+var      = construction.trees_around_snow_line_enabled
+def      = true
+cat      = SC_BASIC
+patxname = ""everest_treeline.construction.trees_around_snow_line_enabled""
+
 [SDT_VAR]
 base     = GameSettings
 var      = construction.trees_around_snow_line_range
 type     = SLE_UINT8
-def      = 0
-min      = 0
-max      = 15
-cat      = SC_BASIC
-
-[SDT_VAR]
-base     = GameSettings
-var      = construction.no_trees_on_this_level
-type     = SLE_UINT32
-def      = 0
-min      = 0
-max      = UINT32_MAX
+def      = 8
+min      = 1
+max      = 20
 cat      = SC_BASIC
+patxname = ""everest_treeline.construction.trees_around_snow_line_range""
 
 [SDT_VAR]
 base     = GameSettings