Change `data_path` from an attribute to a method

[hafen]

I think a nice practical function for getting local paths for data resources would be to change data_path from an attribute to a method.

The method would work as follows:

• If you call p.data_path(), it returns /full/path/to/project/data
• If you call it with an argument of a name or URI, it will return the path to that file. For example, p.data_path("syn:syn18670920") would return full/path/to/project/data/file_associated_with_syn18670920.ext

[pcstout]

@hafen How about a new method called data_path_for(remote_uri)?

[hafen]

I'm not sure what I think about data_path_for(). Maybe get_data_path().