Introduce new model for future remodeling of split transactions.

app/User.php

@@ -42,6 +42,7 @@ use FireflyIII\Models\Rule;
 use FireflyIII\Models\RuleGroup;
 use FireflyIII\Models\Tag;
 use FireflyIII\Models\Transaction;
+use FireflyIII\Models\TransactionGroup;
 use FireflyIII\Models\TransactionJournal;
 use Illuminate\Database\Eloquent\Relations\BelongsToMany;
 use Illuminate\Database\Eloquent\Relations\HasMany;
@@ -320,6 +321,17 @@ class User extends Authenticatable
         return $this->hasMany(Tag::class);
     }
 
+    /**
+     * @codeCoverageIgnore
+     * Link to transaction groups.
+     *
+     * @return HasMany
+     */
+    public function transactionGroups(): HasMany
+    {
+        return $this->hasMany(TransactionGroup::class);
+    }
+
     /**
      * @codeCoverageIgnore
      * Link to transaction journals.