rootio: implement Create function to create new ROOT file

package rootio

func Create(fname string) (*File, error) { ... }

Get acquainted with ROOT/C++:

inspect + understand:
- TFile::TFile, with options == "CREATE" || "RECREATE" || "NEW"
- TFile::Open, with options == "CREATE" || "RECREATE" || "NEW"
inspect + understand TFile::Init(create=true)
inspect + understand TDirectoryFile::FillBuffer
inspect + understand TFile::Close
insepct + understand TFile::WriteStreamerInfo

once that's done:

flesh out func (f *File) writeHeader() error (following TFile::WriteHeader
implement func (f *File) writeStreamerInfo() error (following TFile::WriteStreamerInfo)
improve TestCreateEmptyFile to create, close and then re-open empty file

ROOT I/O main classes