Remove fftw dependencies in tenor/utils.hpp

experimental/include/scalfmm/utils/tensor.hpp

@@ -5,18 +5,13 @@
 #ifndef SCALFMM_UTILS_TENSOR_HPP
 #define SCALFMM_UTILS_TENSOR_HPP
 
-#include <atomic>
 #include <bits/c++config.h>
-#include <complex>
-#include <deque>
-#include <functional>
-#include <iterator>
-#include <mutex>
-#include <numeric>
+#include <utility> // for std::forward
+#include <iterator> // std::begin
+#include <numeric> // for std::iota
 #include <type_traits>
+#include <stdexcept>
 
-#include <fftw3.h>
-#include <utility>
 #include <xsimd/memory/xsimd_load_store.hpp>
 #include <xtensor/xadapt.hpp>
 #include <xtensor/xarray.hpp>