rootio: create write benchmarks

[sbinet]

once we have write capabilities, we should have go test benchmarks:

• [ ] write 1 branch with 1 float64
• [ ] write 10 branches with 1 float64
• [ ] write 1 branch with [10]float64
• [ ] write 1 branch with []float64 of len 10
• [ ] write 1 branch with struct { F64 float64 }
• [ ] write 1 branch with struct { F64s [10]float64 }
• [ ] write 1 branch with struct { F64s []float64 }