Add state version implementations for neuron and synapse models

This commit implements state versions of several neuron and synapse models
using the brainstate framework, following BrainPy v2.7+ architecture.

Neuron Models Added:
- LIF (Leaky Integrate-and-Fire) neurons with multiple variants:
  * LIF: Basic LIF neuron with exponential synaptic input
  * LifRef: LIF with refractory period
  * ExpIF: Exponential Integrate-and-Fire
  * ExpIFRef: ExpIF with refractory period
  * AdExIF: Adaptive Exponential Integrate-and-Fire
  * AdExIFRef: AdExIF with refractory period
  * QuaIF: Quadratic Integrate-and-Fire
  * QuaIFRef: QuaIF with refractory period
  * AdQuaIF: Adaptive Quadratic Integrate-and-Fire
  * AdQuaIFRef: AdQuaIF with refractory period
  * GifRef: Generalized Integrate-and-Fire with refractory

- Izhikevich neuron model with variants:
  * Izhikevich: Basic Izhikevich neuron
  * IzhikevichRef: With refractory period

- Hodgkin-Huxley (HH) neuron model:
  * HH: Classic Hodgkin-Huxley model with Na+ and K+ channels

Synapse Models Added:
- BioNMDA: Biological NMDA receptor with second-order kinetics
  * Implements two-state cascade dynamics (x and g variables)
  * Slower rise time compared to AMPA (biologically realistic)
  * Comprehensive documentation with mathematical formulation

Testing:
- Comprehensive test suites added for all models
- AMPA and GABAa synapse tests added
- All tests passing with proper unit handling

Key Features:
- Uses brainstate ecosystem (HiddenState, ShortTermState, LongTermState)
- Proper unit support with brainunit
- Exponential Euler integration for numerical stability
- Batch processing support
- Consistent API design across all models

Files Modified:
- brainpy/state/_lif.py: Added LIF variants
- brainpy/state/_izhikevich.py: Added Izhikevich variants (new file)
- brainpy/state/_hh.py: Added HH model (new file)
- brainpy/state/_synapse.py: Added BioNMDA model
- brainpy/state/_synapse_test.py: Added comprehensive tests
- brainpy/state/_lif_test.py: Added LIF tests
- brainpy/state/__init__.py: Updated exports
- brainpy/dyn/neurons/lif.py: Minor documentation updates

Author: 932179209

brainpy/dyn/neurons/lif.py

@@ -857,6 +857,10 @@ class ExpIF(ExpIFLTC):
            conductance-based synaptic drive." Physical Review E 76, no. 2 (2007): 021919.
     .. [5] https://en.wikipedia.org/wiki/Exponential_integrate-and-fire
 
+    .. seealso::
+
+       :class:`brainpy.state.ExpIF` provides the state-based formulation of this neuron.
+
     **Examples**
 
     There is a simple usage example::
@@ -978,6 +982,10 @@ class ExpIFRefLTC(ExpIFLTC):
            conductance-based synaptic drive." Physical Review E 76, no. 2 (2007): 021919.
     .. [5] https://en.wikipedia.org/wiki/Exponential_integrate-and-fire
 
+    .. seealso::
+
+       :class:`brainpy.state.ExpIFRef` provides the state-based formulation of this neuron.
+
     **Examples**
 
     There is a simple usage example::
@@ -1319,6 +1327,10 @@ class AdExIFLTC(GradNeuDyn):
            inputs." Journal of Neuroscience 23.37 (2003): 11628-11640.
     .. [2] http://www.scholarpedia.org/article/Adaptive_exponential_integrate-and-fire_model
 
+    .. seealso::
+
+       :class:`brainpy.state.AdExIF` provides the state-based formulation of this model.
+
     **Examples**
 
     An example usage:

brainpy/state/__init__.py

@@ -22,6 +22,10 @@
 from ._inputs import __all__ as inputs_all
 from ._lif import *
 from ._lif import __all__ as neuron_all
+from ._izhikevich import *
+from ._izhikevich import __all__ as izh_all
+from ._hh import *
+from ._hh import __all__ as hh_all
 from ._projection import *
 from ._projection import __all__ as proj_all
 from ._readout import *
@@ -36,9 +40,9 @@
 from ._synouts import __all__ as synout_all
 from .. import mixin
 
-__main__ = ['version2', 'mixin'] + inputs_all + neuron_all + readout_all + stp_all + synapse_all
+__main__ = ['version2', 'mixin'] + inputs_all + neuron_all + izh_all + hh_all + readout_all + stp_all + synapse_all
 __main__ = __main__ + synout_all + base_all + exp_all + proj_all + synproj_all
-del inputs_all, neuron_all, readout_all, stp_all, synapse_all, synout_all, base_all
+del inputs_all, neuron_all, izh_all, hh_all, readout_all, stp_all, synapse_all, synout_all, base_all
 del exp_all, proj_all, synproj_all
 
 if __name__ == '__main__':