WIP,ENH: MultiPlotter improvements

[GuillaumeFavelier]

This PR adds a margin parameter to the MultiPlotter:

margin=True	margin=False

import pyvista as pv
from pyvistaqt import MultiPlotter

mp = MultiPlotter(nrows=2, ncols=2, menu_bar=False, toolbar=False, margin=False)
mp[0, 0].add_mesh(pv.Sphere())
mp[0, 1].add_mesh(pv.Cylinder())
mp[1, 0].add_mesh(pv.Cube())
mp[1, 1].add_mesh(pv.Cone())

---

ToDo:

• Testing/Coverage

[codecov]

Codecov Report

Merging #90 (791cbf0) into main (7157bbb) will decrease coverage by 0.06%.
The diff coverage is 97.56%.

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
- Coverage   97.22%   97.15%   -0.07%     
==========================================
  Files           8        8              
  Lines         648      669      +21     
  Branches       90       95       +5     
==========================================
+ Hits          630      650      +20     
- Partials       18       19       +1     

[GuillaumeFavelier]

Custom margin would be interesting to have? If not, it's ready to go on my end.

[GuillaumeFavelier]

I'll continue working on this :)