Restrict <sig-block> and <sig>

[obuchtala]

JATS 1.1 allows too many degree of freedoms

• <sig-block> can be used like <sig>; better to allow only one pattern <sig-block><sig></sig></sig-block>
• can contain <contrib> which should better be defined in the front-matter
• examples let assume, that this is intended to include signatures together with descriptions

In general this part seems to be a bit unrestricted or inconsistent: should be a <sig> element a container with text blocks and graphics, or a block-element with a defined structure?