Replace Kokkos::Timer by a builtin one based on std::chrono

This reduces a bit the Kokkos adherence

src/utils/Timer.hpp

 #ifndef TIMER_HPP
 #define TIMER_HPP
 
-#include <Kokkos_Timer.hpp>
+#include <chrono>
+#include <iostream>
 
-using Timer = Kokkos::Timer;
+class Timer
+{
+ public:
+  enum class Status
+  {
+    running,
+    paused,
+    stopped
+  };
+
+ private:
+  std::chrono::time_point<std::chrono::high_resolution_clock> m_start;
+  std::chrono::duration<double> m_elapsed_sum;
+
+  Status m_status;
+
+ public:
+  Status
+  status() const
+  {
+    return m_status;
+  }
+
+  double
+  seconds() const
+  {
+    switch (m_status) {
+    case Status::running: {
+      return (m_elapsed_sum + std::chrono::duration<double>{std::chrono::system_clock::now() - m_start}).count();
+    }
+    case Status::paused:
+    case Status::stopped: {
+      return m_elapsed_sum.count();
+    }
+      // LCOV_EXCL_START
+    default: {
+      return 0;
+    }
+      // LCOV_EXCL_STOP
+    }
+  }
+
+  friend std::ostream&
+  operator<<(std::ostream& os, const Timer& timer)
+  {
+    os << timer.seconds() << 's';
+    return os;
+  }
+
+  void
+  reset()
+  {
+    m_start       = std::chrono::high_resolution_clock::now();
+    m_elapsed_sum = std::chrono::duration<double>::zero();
+  }
+
+  void
+  stop()
+  {
+    m_start       = std::chrono::high_resolution_clock::now();
+    m_elapsed_sum = std::chrono::duration<double>::zero();
+    m_status      = Status::stopped;
+  }
+
+  void
+  pause()
+  {
+    if (m_status == Status::running) {
+      m_elapsed_sum += std::chrono::high_resolution_clock::now() - m_start;
+      m_start  = std::chrono::high_resolution_clock::now();
+      m_status = Status::paused;
+    }
+  }
+
+  void
+  start()
+  {
+    switch (m_status) {
+    case Status::running: {
+      return;
+    }
+    case Status::paused:
+    case Status::stopped: {
+      m_start  = std::chrono::high_resolution_clock::now();
+      m_status = Status::running;
+    }
+    }
+  }
+
+  Timer& operator=(const Timer&) = default;
+  Timer& operator=(Timer&&) = default;
+
+  Timer(const Timer&) = default;
+  Timer(Timer&&)      = default;
+
+  Timer()
+    : m_start{std::chrono::high_resolution_clock::now()},
+      m_elapsed_sum{std::chrono::duration<double>::zero()},
+      m_status{Status::running}
+  {}
+
+  ~Timer() = default;
+};
 
 #endif   // TIMER_HPP