Fix for Issue 19: Variable record length support

This is intended to fix issue #19 (closed) where fixhdr was breaking mseeds with variable record length. Now when mseeds with variable record length are scanned, fixhdr will ask the user to select the mseeds they want to convert into fixed record length. Selected files will then be fixed with truncate -cs %4096 [filename], re-scanned, and added to the trace db to be modified.