Add the option to manually define the simulation start when calibrating a model

[twallema]

• I have checked to ensure there aren't other open Pull Requests for the same update/change
• I have updated the documentation accordingly.

Describe your fixes/additions/changes

The class log_posterior_probability() now has an input argument start_sim you can provide upon initialization. Prior, class just sought the earliest start time/date in the provided datasets and used that as the start of the simulations, which is quite limiting.