mstump: description of idx in private functions

[NimaSarajpoor]

In module mstump.py:

The function _mstump calls the function _compute_multi_D which has the argument idx with the following description:

idx : int
        The row index in `T`

The function _mstump also calls the function core._compute_multi_PI that has the same argument idx with the same description.

However, I think this description might be incorrect. If T is multi-dimensional, i.e. 2D numpy array, where each row corresponds to one time series, then the description of idx should be:

the column index of `T`

This is based on what I can see, regarding idx, in the following lines of code:

https://github.com/TDAmeritrade/stumpy/blob/76cb9806dec2d32f80ff20a5a3c484c34b7a54bf/stumpy/mstump.py#L800-L814

[seanlaw]

I think you're right. Though, I think it's more like:

idx : int
    The subsequence index for the ith time series, `T[i]`

Is this better?

[NimaSarajpoor]

Yeah... I think it is clearer as it reflects a subsequence (that starts at idx)