Add code for EnstoreFileReader and EnstoreReadSession.
Summary
This allows us to read back Enstore tapes. It also fixes a small bug introduced in !2 (merged) which was looking only for the OSM "hex" file size. There is one small change to the base code class to allow the reader-flavor to control the positioning method since Enstore has no idea of block IDs.
References
Fixes #89 (closed) (1272 was the issue in the old repo).