fix for being unable to read root group of hdf5

[neptunes5thmoon]

Currently reading an hdf5 file directly, i.e. read("myfile.h5") throws an error:
ValueError: Unable to synchronously create group (name parameter cannot be an empty string)
I expect it to return the HDF5 group at the file's root.

This PR:

• adds testing for the above
• changes tests for other file formats to use access from core (so that it's before path splitting)
• changes split_by_suffix to return "/" as the subpath if the path ends in one of the known suffixes
• changes the test for path splitting to account for the change in behavior

[d-v-b]

Naming the root group "/" is funny behavior from hdf5 -- "/" is not the name of any node but rather the separator character used to join node names, and thus the empty string "" is the "real" name of the root group. I think we can handle this quirk in the hdf5 reading logic, instead of higher up in the stack. I will get a PR shortly that does this.

[neptunes5thmoon]

closing in favor of #85