Match length of strings when comparing file extensions

This will prevent, for example, a file extension ".e" from matching ".exo" or ".ex2" in pqCMBModelManager::fileModelSessions(). This is part of a small fix to load .e files automatically, instead of displaying a selection list with 2 copies of the Exodus reader to pick from. Combined with the smtk:legacy-files branch, this should resolve issue #117 (closed) (Multiple Exodus Readers).