// Here is the file header format as of release 3.02.06.  It is never compressed.
// -----------------------------------
// byte  0->3  "root"      = Identifies this file as a ROOT file.
//       4->7  Version     = File format version                        TFile::fVersion
//                         |  (10000*major+100*minor+cycle (e.g. 30203 for 3.2.3))
//       8->11 BEGIN       = Byte offset of first data record (64)      TFile::fBEGIN
//      12->15 END         = Pointer to first free word at the EOF      TFile::fEND
//                         | (will be == to file size in bytes)
//      16->19 SeekFree    = Byte offset of FreeSegments record         TFile::fSeekFree
//      20->23 NbytesFree  = Number of bytes in FreeSegments record     TFile::fNBytesFree
//      24->27 nfree       = Number of free data records
//      28->31 NbytesName  = Number of bytes in TKey+TNamed for TFile at creation TDirectory::fNbytesName
//      32->32 Units       = Number of bytes for file pointers (4)      TFile::fUnits
//      33->36 Compress    = Zip compression level (i.e. 0-9)           TFile::fCompress
//      37->40 SeekInfo    = Byte offset of StreamerInfo record         TFile::fSeekInfo
//      41->44 NbytesInfo  = Number of bytes in StreamerInfo record     TFile::fNbytesInfo
//      45->63             = Unused??