Add Hdf5 serialization functions for read and write data objects

[chenyangkang]

PR for issue #220

[chenyangkang]

@ethanplunkett Hi Ethan, can we consider merging this after merging the other PR? I'm working on combining the multi-source data and these functions seem to be essential if we want them in hdf5 format. Please let me know if there are any concerns.

[chenyangkang]

@ethanplunkett Revised based on you suggestions:

1. add identical check for object read and write. The objects should be exactly the same after dumping and reloading.
2. replace the read and write function for the BirdFlowRoutes and Routes with a generic function.
3. Lint.

[ethanplunkett]

@chenyangkang I updated the tests for serialization:

• Use a temp directory to write file
• Properly test (and now fail) writing NULL values

[ethanplunkett]

I fixed the tests so it's actually testing with a NULL list, but now it's failing. I think we should just drop those tests as I'm not aware of anytime that we'll be writing NULL values.