Merge FactorRange when linking HoloViews axes

[philippjfr]

Fixes #798

[codecov]

Codecov Report

Merging #968 into master will increase coverage by 0.11%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #968      +/-   ##
==========================================
+ Coverage    85.2%   85.31%   +0.11%     
==========================================
  Files          96       96              
  Lines       10757    10762       +5     
==========================================
+ Hits         9165     9182      +17     
+ Misses       1592     1580      -12

Impacted Files	Coverage Δ
panel/pane/holoviews.py	84.55% <60%> (-0.32%)	⬇️
panel/pane/vtk/vtk.py	53.43% <0%> (+1.05%)	⬆️
panel/interact.py	76.07% <0%> (+2.5%)	⬆️
panel/util.py	91.79% <0%> (+3.73%)	⬆️

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