add support for reading NWB files

[stephprince]

In addition to writing data to NWB format, the ability to read files back in from other formats is important for acquisition systems to allow users to play back data.

We have discussed this as an important feature to target after the first release. The scope will likely be limited such that only data types produced by that acquisition system will be able to be read back in (e.g. not reading all datatypes available in any NWBFile).