Fix snapping

[ericpre]

Fix regression introduced in #2686: snapping was call too early - before the widget values were set and it was snapping at the default values, typically 0.
Close #2720.

Progress of the PR

• snap widget after the widget values have been set,
• Add tests,
• ready for review.

[codecov]

Codecov Report

Merging #2721 (241c253) into RELEASE_next_patch (b77cc79) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@                  Coverage Diff                   @@
##           RELEASE_next_patch    #2721      +/-   ##
======================================================
+ Coverage               77.21%   77.24%   +0.02%     
======================================================
  Files                     202      202              
  Lines                   30035    30035              
  Branches                 6559     6559              
======================================================
+ Hits                    23193    23201       +8     
+ Misses                   5091     5086       -5     
+ Partials                 1751     1748       -3     

Impacted Files	Coverage Δ
hyperspy/axes.py	86.34% <100.00%> (+0.26%)	⬆️
hyperspy/drawing/_widgets/line2d.py	54.29% <100.00%> (ø)
hyperspy/roi.py	82.07% <100.00%> (+0.49%)	⬆️
hyperspy/drawing/widget.py	71.81% <0.00%> (+0.58%)	⬆️

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[ericpre]

The failure of the test suite are related to recent releases of dependencies and not to this PR.

[ericpre]

Rebased to include the changes the test suite fixes from #2722.

[jlaehne]

Verified that error in #2720 is indeed fixed.