Add support for reading ensight transient single-file format

A number of changes in how ensight reading is done to accommodate the ensight transient single-file format. All changes made such that reading with "regular" ensight files continues to work as before.

Added the framework and provisioning for writing ensight transient single-file format as well, but has not yet been plumbed all the way through.

OFstream now supports append with file rewriting (APPEND_ATE mode), which is requisite for ensight single-file writing but which is also generally useful to have anyhow.