Add flags to chose inputs for model

[NathanHenderson]

Possible flags:

-input input_choice : where input choice would be HCS (Human coordinate system), GRADIENTS or ORIGINAL (No normalisation)

-weights boolean_value : True to use the weights given by openpose, False to discard them.

-expand_set [reflect, scale]: True or False for each to reflect and scale.

[joshbrun]

@NathanHenderson Is this still valid?

[NathanHenderson]

@joshbrun It would be a nice feature to have, but not needed.