Added index to Observation dates.

backend/src/main/kotlin/uk/co/neviyn/observationdatabase/Entity.kt

@@ -40,6 +40,7 @@ enum class RatingCategory {
 }
 
 @Entity
+@Table(indexes = [Index(name = "dateIndex", columnList = "date")])
 data class Observation(
         @Id
         @GeneratedValue(strategy = GenerationType.IDENTITY)
@@ -47,7 +48,7 @@ data class Observation(
         @JsonIgnore
         @ManyToOne
         val site: Site,
-        @Column(nullable = false)
+        @Column(nullable = false, name = "date")
         val date: LocalDate,
         @Column(nullable = false)
         val type: TrainingType,