Trendline options

[nicolaskruchten]

Refactors trendline code to extract a common interface (an ugly one!), including an options dict, which enables new functions like rolling mean (needed an argument for the window size at least!) and OLS options like log_x and add_constant, plus a configurable LOWESS frac parameter.

Code PR

• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• For a new feature, I have added documentation examples in an existing or
  new tutorial notebook (please see the doc checklist as well).
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).