external: Add poolSTL as alternative for parallel sorting.

Add poolSTL to external dependencies. This is an alternative implementation of the parallel stl algorithms and is meant as a replacement for libTBB.

algo/base/compat/Algorithm.cxx

+/* Copyright (C) 2024 FIAS Frankfurt Institute for Advanced Studies, Frankfurt / Main
+   SPDX-License-Identifier: GPL-3.0-only
+   Authors: Felix Weiglhofer [committer] */
+
+#include "Algorithm.h"
+
+
+task_thread_pool::task_thread_pool& cbm::algo::GetGlobalSTLThreadPool()
+{
+  static task_thread_pool::task_thread_pool pool;
+  return pool;
+}