Redo: Issue an error/warning when the source field of a rule is not a list of "type name"

Only complain when the rule has a function/code and not when it is only
an 'attribute' rule.

Extend the information printed in case of rule problems:

WARNING: IO rule for class MyClass - type required when listing a rule's source: source="fTwo"
The following rule has been omited:
   read sourceClass="MyClass" targetClass="MyClass" source="fTwo" target="fTwo" code="{ fTwo = -1; newObj->reset1(); }";