Updates

[chaoming0625]

• New kinds of synapse models have been defined in brainpy.syn module. This module provides models that can be called as a deep learning layer.
• Some previous neuron models can also be called as deep learning layers. Simply define these neuron models as NeuonModel(... input_var=False).
• share context and delay are migrated into brainpy.dyn, because these are used specialied for dynamical systems.
• Fix several bugs.
• Update examples.

[ztqakita]

Massive update! Great work!