Make jebl2 SequenceType and State easy to extend

[walterxie]

@rambaut
How are you? @alexeid and I are working on a project depending on jebl2. I proposed few changes to make the code easy to extend. It will be much appreciated if you would have a look.

1. make State constructor not "package private", so we can use our package name;
2. change some fields and methods to protected in NexusImporter;
3. extract NexusImporter.getSequenceType(String) to allow handling new data types in the subclass;
4. add Binary data type;
5. create SequenceType.Utils.getDataType(String) for Nexus parsing;
6. add new blocks to NexusBlock, which can be used in new implementation or the subclass.