Split into a generic + methods, and an internal calculation function

- `.coef_plot` plots a fit based on a data frame of estimates and uncertainty values
- `coef_plot.*` methods extract the estimates and uncertainty from fitted models to
   pass to the plotting function
- This allows to reuse more code, separate calculations from class specific commands,
  and make it easier to add implementations for new models.
- Currently only `lm` is implemented