Incorrect steps saved with successive model checkpointing

[hayesall]

Running from a checkpoint doesn't properly save the stepsize array to the resulting model file.

For example, learning and checkpointing three times:

-trees 3 // Checkpoint the model
-trees 4 // Checkpoint the model
-trees 5 // Checkpoint the model

Results in:

5
cancer
[1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0]
-1.8
cancer

i.e., a merge occurs instead of an override:

# Length 3 + Length 4 + Length 5
[1.0, 1.0, 1.0], [1.0, 1.0, 1.0, 1.0], [1.0, 1.0, 1.0, 1.0, 1.0]