Update seir

[ConniCia]

Hi all! I've updated the SEIR_model scripts and verified they run correctly.

Main changes are:

• correcting the formula to compute parameter beta
• adding a condition in the MCMC so that it doesn't fail whenever the loglikelihood is NA, but rejects the proposed parameter values and moves on to the next iteration

Other changes are only about formatting.

[ConniCia]

In the additional commits I've

• fixed a wrong file name
• allowed the code to pick a seeding time and adjusted plotting functions for it
• made some adjustments to the README

[ConniCia]

Minor adjustments made. Code tested.