Fix 'Complex Method' issue in palmer.py, _pdsi_from_zindex(() function

[monocongo]

Investigate whether or not this code's logic follows closely what is described in the relevant literature (Palmer 1965, Wells 2004).

If so then look into how the code can be simplified and/or modularized further. Use vectorization where possible. Reduction of cyclomatic complexity encouraged where reasonable.

If not then address the divergence from accepted/established algorithm, remeasure, repeat.

CodeFactor found an issue: Complex Method

It's currently on: palmer.py:989-1169 Commit fdd08e5ef1ba0cec14a5c9fa885c34e19f4ffa98