Feature/pivottable conditional formatting (#1598)

* WIP:Added support for conditional formatting in pivot tables

* Added new validations and tests

* WIP:Added read test

* Deleting conditional formattings for deleted values when refreshing a pt

* Fixed failing tests

* Renamed classes and enums. Refactoring

Author: JanKallman

src/EPPlus/ConditionalFormatting/ExcelConditionalFormattingCollection.cs

@@ -583,8 +583,9 @@ public void RemoveByPriority(
             {
                 Remove(RulesByPriority(priority));
             }
-            catch
+            catch(Exception ex)
             {
+                throw new InvalidOperationException($"Could not remove item with priority {priority}", ex);
             }
         }
 
@@ -1877,5 +1878,17 @@ public IExcelConditionalFormattingDataBarGroup AddDatabar(string Address, Color
 
             return dataBar;
         }
+
+        internal IExcelConditionalFormattingRule GetByPriority(int priority)
+        {
+            foreach (var rule in _rules)
+            {
+                if(rule.Priority == priority)
+                {
+                    return rule;
+                }
+            }
+            return null;
+        }
     }
 }