[WIP] allow JSON data descriptor to iterate over series of JSON files

The JSON Data Descriptor can now take in a series of files. It does this by basically operating on a current file, and then having a mechanism to go to the next file. So, as an iterable, the datadescriptor goes over every file. But, if you just do something like:

```Python
dd = JSON_DDesc([self.json_file, self.json_file2], schema=json_schema)
ddesc_as_py(dd)
```

You only get the stuff in the current file (in this case, the first file). So, the question that I have is whether or not asking for the full array should go and read every file given to the data descriptor constructor. Very interested in feedback.