Sporadic NWB read error can occur during EphysSession object construction

[vijayiyer05]

This error was first reported by @kzhou085.

It can occur during the repeated calls to h5info in bot.nwb.nwb_ephys while accessing a cached and uncorrupted NWB file linked to an EphysSessoin object.

Some initial observations:

• May be more likely to occur when disk space is low
• The multiple repeated calls may be possible to reduce, e.g. by making Session objects into singletons (Consider singleton implementation for item objects, i.e. one object per id.  #129)