Clarify eprop iaf psc delta purpose

[akorgor]

This PR aims to

• clarify the purpose of the eprop_iaf_psc_delta neuron model as the iaf_psc_delta neuron model endowed with e-prop plasticity.
• reduce the differences between the two models.
• make pre_run_hook, init_buffers, and update private functions in all e-prop models as in the standard iaf_psc_delta .
• make e-prop related functions private.
• remove doxygen comment of get_eprop_isi_trace_cutoff since it is inherited from node.h.
• add _bsshslm_2020 to the tutorials headers to make them easily distinguishable in the compiled documentation.
• make the compute gradient definition consistent.