Better API for specifying return data of signals

The current rdrecord API is rather confusing when dealing with more complex records/signals. Namely: multi-frequency signals.

We should create a better API to allow users to:

• Specify/know the exact return types/dimensions of the signals being read
• Deal with single and multi-frequency records consistently.

In addition, the current API is also limiting when reading single-frequency records, as there is no way for users to explicitly specify whether they want 1d or 2d numpy arrays.

We current have: https://github.com/MIT-LCP/wfdb-python/blob/0d42dfb4b2946625f00cbf500d830d374a201153/wfdb/io/record.py#L1715 where the only parameter that influences the return type is: smooth_frames

We've chatted about the issue here, where I suggested a new set of params:
https://github.com/MIT-LCP/wfdb-python/pull/313#issuecomment-1058363761

What do people think of the suggestion of having two params: return_dims and smooth_frames? return_dims will apply for both single and multi-frequency signals, and smooth_frames will apply only to multi-frequency signals. Given the combination of the params, the return value of the signals will be:

(1, True): List of 1d arrays. Guaranteed to be same length.
(1, False): List of 1d arrays. Not guaranteed to be same length for multi-frequency signals.
(2, True): Single 2d array.
(2, False): Illegal combination.

首先阅读 wfdb/io/record.py 第1715行附近的内容以及链接的 pull request 讨论，因为拟议的参数和兼容性问题仍在讨论中。完成的标准是确定并记录 return_dims 和 smooth_frames 下单频与多频信号的返回形式，并一致地处理非法组合。