LGTM.com - false positive: Iteration over sequence, of class list, may also iterate over string.

**Description of the false positive**

Here the type is checked before before the for loop. If it is not a list of tuple, then it is set to list.

**URL to the alert on the project page on LGTM.com**

https://lgtm.com/projects/g/spectrochempy/spectrochempy/snapshot/e28b7624d20e89aa1d5ff919190dff06e445e8c4/files/spectrochempy/core/dataset/ndio.py?sort=name&dir=ASC&mode=heatmap#x5f9ca077da72a4b6:1