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.cpp

@@ -0,0 +1,26 @@
+#include "DurationWrapper.h"
+#include <cmath>
+
+namespace sls{
+
+    DurationWrapper::DurationWrapper(double seconds){
+        ns_tick = std::round(seconds*1e9);
+    }
+
+    uint64_t DurationWrapper::count() const{
+        return ns_tick;
+    }
+
+    void DurationWrapper::set_count(uint64_t ns_count){
+        ns_tick = ns_count;
+    }
+
+    bool DurationWrapper::operator==(const DurationWrapper& other)const{
+        return ns_tick == other.ns_tick;
+    }
+
+    double DurationWrapper::total_seconds()const{
+        return static_cast<double>(ns_tick)/1e9;
+    }
+
+}