Theory class: add method d_nu and fix computation of gain matrix, rename stability_matrix to gain_matrix

Fix calculation of stability matrix in Theory class, formatting in MultiAreaModel class, remove deprecated functions from analysis_helpers

Enh/universal spike detector

Properly raise a NotImplementedError if trying to use the not yet implemented stabilization procedure and test for this

Exclude parameters with explicit paths from label determination of classes because they might differ between systems

RE-integrated SLN-R-Fit into scripts, but remove SLN_logdensities.R and deactivate SLN Fit for now because of cell density data copyright issue

Implement for flexible option for initial rates in theory, add parameters to default_params, improve documentation of theory class and theory params in default_params

Rename viscortex_processed_data.json to viscortex_processed_data_backup.json and store only necessary predefined data