fit_model: Fit Poll Pooling Model
In jackobailey/PollBasePro: Data and Functions for Studying British Politics

This function provides a simple way to fit a variant of Simon Jackman's "polling the polls" model.

1	fit_model(data, init, final, party, alpha_init, alpha_final, refresh = 0)

`data`	Raw pollbase data.
`init`	Initial election date.
`final`	Final election date.
`party`	A string reflecting a party. Either "con", "lab", or "lib".
`alpha_init`	The party's vote share at the initial election.
`alpha_final`	The party's vote share at the final election.
`refresh`	How often to report model iterations. Defaults to 0.