added to_string method to matrix

tests/UnitTestMatrix.cpp

@@ -0,0 +1,15 @@
+//
+// Created by vlad on 5/18/2024.
+//
+#include <gtest/gtest.h>
+#include <uml/matrix.h>
+#include <print>
+TEST(UnitTestMatrix, ToString)
+{
+    uml::matrix matrix(2, 2);
+    matrix.set(1.1);
+    const auto str = matrix.to_string();
+
+    std::cout << str;
+
+}