remove iswt2's restriction on non-square inputs

[grlee77]

iswt2 enforces square shaped inputs (this is even documented in a comment), but that seems to be unnecessary. This PR just removes the restriction and adds a basic test case with rectangular inputs.

closes #368

[codecov-io]

Codecov Report

Merging #369 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #369   +/-   ##
=======================================
  Coverage   83.58%   83.58%           
=======================================
  Files          21       21           
  Lines        3332     3332           
  Branches      567      567           
=======================================
  Hits         2785     2785           
  Misses        483      483           
  Partials       64       64

Impacted Files	Coverage Δ
pywt/_swt.py	86.77% <0%> (ø)	⬆️

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