Template Constraint and Data-model Doc Refactor

    Apache IoTDB supports Measurement Template feature from version 0.13. The current version allows user to set different templates to different nodes of a path and the nearest template to current node will be valid. 
    This will result in schema and data lost while user set a new template, which doesn't contain schemas in the existing upper template.
    Therefore, a new constraint will be introduced. If a template has been set to a node, it will be forbidden to set any template to the ancestors or descendants of this node, like the constraint of storage group.