DEPR: Unify parameter naming

[derb12]

Description of the problem/new feature

There are a few places where methods use different parameter names that mean the same thing. For consistency's sake, it'd be nice to pick one keyword and deprecate all other usages.

• loess uses symmetric_weights, where I think all other places like mixture_model and beads just use symmetric
  • penalized_poly could also get symmetric added such that it's cost_function term can be a simple 3-option enum (goldindec could likewise be updated), although I'm not entirely sold on the idea
• rubberband uses segments while golotvin and peak_filling use sections
• snip and swima use max_half_window; the meaning's technically different than just half_window, but I think it's close enough to justify changing to half_window since the later term is used everywhere else for smoothers
• custom_bc should use lam_smooth instead of lam

And while I'm at it, since v1.3 is shaping up to be the "deprecation version", I should rename loess and penalized_poly:

• loess should be renamed to lowess; way back when I implemented it, I thought "loess" just meant "lowess" for higher polynomial orders, but it's an entirely separate algorithm; naming it loess dilutes Cleveland's algorithm naming scheme, which wasn't my intention
  • should also be moved to smooth since labeling it as a polynomial is a bit of a misnomer
• penalized_poly should be renamed to robust_poly; again, a misunderstanding on my part; a penalized polynomial would make one think of Tikhonov regularization rather than what penalized_poly does, which is more akin to robust regression/M-estimation.