New simulations

[jgallowa07]

This PR:

• Addresses the simulations outlined in Simulations outline #125.
• Adds a fully polished notebook that we've added to the documentation with simulated data, fitting, and cross-validation as requested in Add Simulations and Spike analysis to documentation #132.
• Removes Jax 64-bit precision accuracy. I've noted that it has very little impact on the simulations, and should thus be off by default to follow JAX best-practices
• It's adds new functionality ModelCollection.add_validation_loss and Model.get_df_loss methods to streamline cross-validation analyses.
• Testing for the new validation methods.
• Specifies a zero beta ridge penalty when computing the Model.loss, this has no effective behavior impact, mainly for code-readability.
• fixes other minor typos in the code I came across.