Improved handling of padding, strides and initialisation

Added

• co.Delay handling for padding.
• Handling of initialisation and strides in containers

Changed

• co.Conv build_from behavior to not change dilation and stride. Argument overload supported instead.
• pad_start and pad_end args to convolution and pooling modules forward_steps.
• Behavior of modules while they initialise. Now, a TensorPlaceholder is passed for initialising steps.

Removed

• Automatic unsqueeze in pooling.