LunaEvent: allow quantity of 0

This is helpful for storing an index

app/src/main/java/it/danieleverducci/lunatracker/entities/LunaEvent.kt

@@ -45,9 +45,9 @@ class LunaEvent: Comparable<LunaEvent> {
             jo.put("type", value)
         }
     var quantity: Int
-        get() = jo.optInt("quantity")
+        get() = jo.optInt("quantity", -1)
         set(value) {
-            if (value > 0)
+            if (value >= 0)
                 jo.put("quantity", value)
         }
     var notes: String

app/src/main/java/utils/NumericUtils.kt

@@ -62,7 +62,7 @@ class NumericUtils (val context: Context) {
 
     fun formatEventQuantity(event: LunaEvent): String {
         val formatted = StringBuilder()
-        if (event.quantity > 0) {
+        if (event.quantity >= 0) {
             formatted.append(when (event.type) {
                 LunaEvent.TYPE_TEMPERATURE ->
                     (event.quantity / 10.0f).toString()