* imc_channel, imc_raw: optimize with pass by reference

* imc_object: asc_time, localtime: make threadsafe
* imc_datatype: satisfy 'rule of two'
* python: remove all unused imports

lib/imc_raw.hpp

@@ -300,7 +300,7 @@ namespace imc
     }
 
     // list a particular type of block
-    std::vector<imc::block> list_blocks(imc::key mykey)
+    std::vector<imc::block> list_blocks(const imc::key &mykey)
     {
       std::vector<imc::block> myblocks;
       for ( imc::block blk: this->rawblocks_ )