Added the donation classes

ref #1

src/models/DistanceDonation.ts

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+import { IsInt, IsNotEmpty, IsObject, IsPositive } from "class-validator";
+import { Donation } from "./Donation";
+import { Runner } from "./Runner";
+
+/**
+ * Defines the DistanceDonation class.
+ * For distanceDonations a donor pledges to donate a certain amount for each kilometer ran by a runner.
+*/
+export class DistanceDonation extends Donation {
+  /**
+   * The donation's associated runner.
+   * Used as the source of the donation's distance.
+   */
+  @IsObject()
+  @IsNotEmpty()
+  runner: Runner;
+
+  /**
+   * The donation's amount donated per distance.
+   * The amount the donor set to be donated per kilometer that the runner ran.
+   */
+  @IsInt()
+  @IsPositive()
+  amountPerDistance: number;
+
+  /**
+   * The donation's amount in cents (or whatever your currency's smallest unit is.).
+   * Get's calculated from the runner's distance ran and the amount donated per kilometer.
+   */
+  public get amount(): number {
+    let calculatedAmount = 0;
+    try {
+      calculatedAmount = this.amountPerDistance * (this.runner.distance / 1000);
+    } catch (error) {
+      throw error;
+    }
+    return calculatedAmount;
+  }
+
+}

src/models/Donation.ts

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+import {
+  IsInt,
+  IsNotEmpty,
+  IsObject
+} from "class-validator";
+import { Donor } from './Donor';
+
+/**
+ * Defines the Donation base calss.
+ * A donation just associates a donor with a donation amount.
+ * The specifics of the amoun's determination has to be implemented in child classes.
+*/
+export abstract class Donation {
+  /**
+   * Autogenerated unique id (primary key).
+   */
+  @IsInt()
+  id: number;
+
+  /**
+   * The donations's donor.
+   */
+  @IsNotEmpty()
+  @IsObject()
+  donor: Donor;
+
+  /**
+   * The donation's amount in cents (or whatever your currency's smallest unit is.).
+   * The exact implementation may differ for each type of donation.
+   */
+  public abstract get amount(): number;
+}

src/models/FixedDonation.ts

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+import { IsInt, IsPositive } from "class-validator";
+import { Donation } from "./Donation";
+
+/**
+ * Defines the FixedDonation entity.
+ * In the past there was no easy way to track fixed donations (eg. for creating donation receipts).
+*/
+export class FixedDonation extends Donation {
+
+  /**
+   * The donation's amount in cents (or whatever your currency's smallest unit is.).
+   */
+  @IsInt()
+  @IsPositive()
+  amount: number;
+}