Change "ODM_CLUSTER" column to "SPLIT_PARAMS"

[coreysnipes]

Currently the ODM_CLUSTER column is just a flag to indicate whether split was used. All current benchmark data is "N".

To support more detailed split/merge info in future benchmarks, I suggest changing the column header to "SPLIT_PARAMS", and changing all existing data to "-" (to indicate that split was not used).

Recommended notation for split params in future data: "sX soY"
Derived from: "--split X --split-overlap Y"

e.g., notation in benchmarks data would be "s200 so150" for --split 200 --split-overlap 150