Wrote component help functions for all core ngc-learn comps

[ago109]

Wrote a ".help()" routine for all components in:

ngclearn.components.neurons.graded
ngclearn.components.neurons.spiking
ngclearn.components.synapses.hebbian (including DenseSynapse in ngclearn.components.synapses)
ngclearn.components.other
ngclearn.components.input_encoders

Help functions return a nested dictionary containing core properties of a component, including its high-level explanation as well as information on its core dynamics (and/or plasticity if applicable) and small description of hyper-parameters. Furthermore, input and output components (and their variable names) are explained/distinguished