Refactoring neuronal and synaptic components

[rxng8]

Refactoring components TODO-list

NOTE: Please don't merge this to the v3 upstream until we finish all TODO items and carefully review the code.

The following files will be refactored

1. Graded Neurons:

• bernoulliErrorCell.py
• gaussianErrorCell.py
• laplacianErrorCell.py
• rateCell.py
• rewardErrorCell.py

2. Patched Synapses:

• hebbianPatchedSynapse.py
• patchedSynapse.py

3. Hebbian Synapses:

• BCMSynapse.py
• eventSTDPSynapse.py
• expSTDPSynapse.py
• hebbianSynapse.py

4. Others:

• expKernel.py

5. Spiking Neurons:

• sLIFCell.py

The following test cases will be modified

1. Graded Neurons:

• test_bernoulliErrorCell.py
• test_gaussianErrorCell.py
• test_laplacianErrorCell.py
• test_RateCell.py
• test_rewardErrorCell.py

This PR should be merged when all items in the checklist are completed and carefully reviewed.