Support minutes in Refdate() function

src/ncfuncs.cpp

@@ -91,6 +91,7 @@ std::tuple<MDateTime, time_t, bool> NCFuncs::Refdate(const MString& refdate)
  auto    ci    = words.begin();
  if(ci != words.end())
  {
+  if(*ci == "minutes") step = 60;
   if(*ci == "hours") step = 3600;
   if(*ci == "days") step = 3600 * 24;
   ci++;