Implemented _intermediate_fields for function fields
Improved degree() for function fields
Add relative vector spaces for function fields
improved docstring of is_RationalFunctionField
Merge branch 'ticket/16523' into develop_staged

Conflicts:
	src/sage/rings/function_field/function_field.py
Merge branch 'develop_staged' into mac_lane

Conflicts:
	src/sage/rings/function_field/function_field.py
	src/sage/rings/function_field/maps.py
removed merge artifacts.
Merge branch 'develop_staged' into mac_lane

Conflicts:
	src/sage/rings/function_field/function_field.py