Changes to yanny to allow for multiple structs and custom comments

A few simple changes to yanny and write_ndarray_to_yanny to allow:

1). Multiple structs to be written to the same file via write_ndarray...
2). A file object to be passed to yanny.write this allows the user to populate the file with whatever comments they want before writing the yanny information.

Sorry that this is on the master branch; I had already made the changes by the time I learned of branch 0.2.2.

I updated the code while trying to not interfere with the rest of yanny as much as possible, so there might be a better way to do this stuff, so I understand if these changes don't get pulled.