Nanosecond times in Python (#522)

* initital implementation

* datetime replaces with sls::Duration in Python C bindings

* using custom type caster

* fix for conversion to seconds

* added set_count in python

* common header for pybind11 includes

authored-by: Erik Frojdh <erik.frojdh@psi.ch>

python/src/DurationWrapper.h

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: LGPL-3.0-or-other
+// Copyright (C) 2021 Contributors to the SLS Detector Package
+#pragma once
+#include <cstdint>
+
+namespace sls{
+/*
+Wrapper for nanoseconds stored in uint64_t, used for conversion between
+std::chrono::nanoseconds and python (float or sls::DurationWrapper)
+*/
+
+class DurationWrapper{
+    uint64_t ns_tick{0};
+
+    public:
+        DurationWrapper() = default;
+        explicit DurationWrapper(double seconds);
+        ~DurationWrapper() = default;
+
+        bool operator==(const DurationWrapper& other) const;
+        uint64_t count() const;
+        void set_count(uint64_t count);
+        double total_seconds() const; 
+};
+
+}