Added OpenMP

Add OpenMP to some operator implementations:

• AvgPooling
• BatchNorm
• ConvDepthWise
• Conv
• Fc
• GlobalAveragePooli‎ng
• MatMul
• MaxPooling
• Softmax

For now, OpenMP is automatically enabled if present on the system.

Example of execution time

On MobileNetv1, in debug.

Without OpenMP

With OpenMP