Update MANIFEST.in to include export template.

MANIFEST.in

 include README.md LICENSE
-recursive-include aidge_core *.py 
+recursive-include aidge_core *.py
 recursive-exclude aidge_core/unit_tests *.py
 
 recursive-include aidge_core/aidge_export_aidge *
+recursive-include aidge_core/export_utils/templates *
+
 recursive-include include *.hpp
 recursive-include src *.cpp
 recursive-include python_binding *.cpp