Support for delta synapse

[chaoming0625]

Please:

• Check for duplicate requests.
• Describe your goal, and if possible provide a code snippet with a motivating example.

The new API in brainpy for synaptic projections does not support Delta synapse model.

This is only supportted in old API version, suck like brainpy.synapses.Delta.

[chaoming0625]

This model is usually used in deep spiking neural networks. Therefore, it is essential to support delta synapse models in the context of new projection APIs.

[chaoming0625]

Solved in #568