Add possibility to check for existence/non-existence of annotations in the CAS and issue a warning

```
Add new methods that allow components to check for the existence/non-existence of certain
types in the CAS.

E.g. tokenizers might check whether a reader has already added tokens.

A possible form would be
  requireExists(Token.class)
  requireNotExists(POS.class)
```

Original issue reported on code.google.com by `torsten.zesch` on 2014-10-15 14:14:18
