Avoid 'hybrid' elements

[obuchtala]

With hybrid elements we mean those nodes which can be used inline as well as
on block-level. Typically, this is due to one of the following reasons:

1. A container element has 'mixed=true' to allow for simple text content

Example: it is allowed to write

  <td>123</td>

but also

  <td><p>123</p><p>456</p></td>

and unfortunately even

  <td>123<p>456</p></td>

2. A structured element uses annotations as fields

Example: <element-citation>

3. A text-node (mixed=true) allows for block-level children elements.

Example: <p> in JATS can contain other block-level elements such as <fig>.

Case 1. can not be solved by changing the schema, due to the lack of expressiveness of XSD. One strategy could be declaring these containers as hybrids explicitly,
but only allow either the content to be phrasing-content, or a sequence of block-level elements, but not mixed.
Alternatively, we are considering to normalise this by treating <td>123</td> as <td><p>123</p></td> internally, and dropping the extra wrapper on export.

Case 2 and 3 should be fixed.