Make the ConlluPOSSampleStream constructor public

The constructor for ConlluPOSSampleStream is currently package private as it does not have a public/private keyword before it. Making it public will allow an application to create that stream for use in training a POS model.

The other conllu stream classes (ConlluStream, ConlluLemmaSampleStream, ConlluTokenSampleStream, ConlluSentenceSampleStream) have public constructors, allowing them to be used in training their respective models.

The ConlluPOSSampleStreamFactory and ConlluLemmaSampleStreamFactory classes are marked as internal use only, and rely on an API suited to the opennlp cli. Note: ConlluSentenceSampleStreamFactory and ConlluTokenSampleStreamFactory are missing an internal use only comment, but should have one.