Start putting together pipeline architecture

[ebrahimebrahim]

Close #35

Start putting together pipeline architecture laid out in #44

• [x] add a placeholder table-accessor method to AbcdEvent
• [x] implement InMemoryBvalResource and BvalResource
• [x] implement the other abstract resources and their InMemory subclasses
• [ ] implement the io.py on-disk subclasses of the resources
• [ ] create conversion functions between in-memory and on-disk versions of all the resources
• [ ] use them to create Dwi-level conversion functions between in-memory and on-disk
• [ ] expose those functions as methods of Dwi
• [ ] unit tests