fpspreadsheet: Implement virtual reading mode for biff8 and biff5 (activated by workbook option boVirtualMode when reading). Add demo_virtualmode_read. Update speed test (factor 2 faster than standard mode, main advantage: no significant memory usage)

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3372 8e941d3f-bd1b-0410-a28a-d453659cc2b4