Accessing properties and data outside of the context manager

[bnlawrence]

We currently believe that it is important for thread safety that not all threads share the same open file. We also currently believe that h5netcdf needs the file open to do anything with an h5netdf variable. While we want that to change, our current workaround will be to allow h5netcdf users with pyfive backends to get access to a pyfive variable instance for data access.

The expected usage needs a test and support within h5d.

This issue has arisen as part of David's tests in #21

[bnlawrence]

@davidhassell If you could see if this solves your problems, and if it does, we can close this issue. All tests pass, including the new one (which failed for data access before adding this).

[bnlawrence]

(The commit above included some pseudo chunking stuff that leaked in from another branch, I removed it in 34a684a)

[bnlawrence]

I think the new tests show this work fine.