M2U:Introduce external variables (exog) to assist in prediction

[oneJue]

This change splits a dataset into target variables and external variables (exog) based on the target_channel configuration. The target_channel determines which columns are treated as target variables.

• Integers (positive or negative) can be used to specify a single column index.
• Lists or tuples containing two integers (e.g., [2, 4]) can be used to select a range of columns.
• If target_channel is set to None, all columns are considered target variables, and the exog part will be set None.

Example:

Given the following configuration:

--strategy-args {"horizon":24,"target_channel":[0,-1,[0,3]]}

In this case:

• target_channel = [0, -1]: Selects the first column (index 0) and the last column (negative index represents counting from the end).
• target_channel = [0, 3]: Selects columns from index 0 to 2 (exclusive of index 3), i.e., the first three columns are used as target variables.

The dataset will be split accordingly into target variables and exog, allowing for efficient time-series prediction tasks.