Merge functions for Layers

[syt123450]

Implement merge function for Tensorspace, support all merge function in keras including:

• Add3d
• Substract3d
• Multiply3d
• Average3d
• Maximum3d
• Concatenate3d
• Dot3d
• Add2d
• Substract2d
• Multiply2d
• Average2d
• Maximum2d
• Concatenate2d
• Dot2d
• Add1d
• Substract1d
• Multiply1d
• Average1d
• Maximum1d
• Concatenate1d
• Dot1d

[syt123450]

User can use merge function like: TSP.add([layer1, layer2], config) to get a MergedLayer