[BUG] Multivariate channel initialisation on v0.2.6

[baraline]

In some case, shapelet initialization for multivariate time series return mostly channel 0. The problem is removed by setting Numba parallel to False.

• Find the origin of the bug in the multivariate initialization with parallel option to True.

[baraline]

The bug is due to the index variables a1/a2 used to affect channel values to the channel parameter array. This operation is not safe for parallel processing, as a1 and a2 can be concurrently modified by each thread.

• Modify indexing scheme for multivariate shapelet transformers to allow for parallel initialisation in a thread-safe manner