Fix for zones reopening in simulation-based constraint mechanism for work/school location choice (#1028)

* addresses issue with zones reopening after being closed in shadow_pricing

* addresses issue with zones reopening after being closed in shadow_pricing

* adding shadow price regression values

* updating change log

---------

Co-authored-by: juangacosta <juan.acosta@rsginc.com>

Author: dhensle

activitysim/abm/tables/shadow_pricing.py

@@ -860,10 +860,20 @@ def update_shadow_prices(self, state):
                 sprice.replace([np.inf, -np.inf], 0, inplace=True)
 
                 # shadow prices are set to -999 if overassigned or 0 if the zone still has room for this segment
-                self.shadow_prices[segment] = np.where(
+                old_shadow_prices = self.shadow_prices[segment].values
+                new_shadow_prices = np.where(
                     (sprice <= 1 + percent_tolerance / 100), -999, 0
                 )
 
+                # the conditions above allow for zones to be reopened, but we want to prevent such behavior
+                new_shadow_prices = np.where(
+                    (old_shadow_prices == -999) & (new_shadow_prices != -999),
+                    old_shadow_prices,
+                    new_shadow_prices,
+                )
+
+                self.shadow_prices[segment] = new_shadow_prices
+
                 zonal_sample_rate = 1 - sprice
                 overpredicted_zones = self.shadow_prices[
                     self.shadow_prices[segment] == -999