minor issue: Out of date docstring.

Description

The sym instantiation parameter of the PowerSeriesProfile is declared to be bool in the class docstring and while it is set to a str in the default values.

What should be the type for this is input?

Proposition

1. Non API breaking: Add some type hint to the __init__ function to clarify the expected usage:

def __init__(self, params=None, modes=None, sym="auto", name=""):

can become

def __init__(self, params: Sequence | None, modes: Sequence | None, sym: bool | Literal["auto", "even"] = "auto", name: str)

And make type checking happy.

2. API simplification (can break dependent use-cases): make the sym input a string input and type hint it as a literal: Literal["odd", "even", "auto"] = "auto"

and update the docstring accordingly 😃.

Happy to open a PR for if you are interested.