v0.5.1-p0

• Add PReLU layer with shared_axes.
• Add compatibility to Eigen 3.2.x.
• Add parametrized ReLU layers, drop relu6.
• Switch from channel_first to channel_last order. This is an API-breaking change:
  • Models converted from .h5 to .json with older versions have to be re-converted.
  • shape3 was dropped, shape_hwc was added instead.
  • tensor3_pos was dropped, tensor3_pos_yxz was added instead.
  • tensor3::get was dropped, tensor3::get_yxz was added instead.
  • tensor3::set was dropped, tensor3::set_yxz was added instead.