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:

stumpy/stumpy/mstump.py

Lines 800 to 814 in 76cb980

	for i in range(d):
	# Numba's prange requires incrementing a range by 1 so replace
	# `for j in range(k-1,0,-1)` with its incrementing compliment
	for rev_j in prange(1, k):
	j = k - rev_j
	# GPU Stomp Parallel Implementation with Numba
	# DOI: 10.1109/ICDM.2016.0085
	# See Figure 5
	if idx % 2 == 0:
	# Even
	QT_even[i, j] = (
	QT_odd[i, j - 1]
	- T[i, idx - 1] * T[i, j - 1]
	+ T[i, idx + m - 1] * T[i, j + m - 1]
	)

[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)