Fix all-triangles checking for polyfilters

[adeak]

We have a few filters that need all-triangle meshes (and this is even documented), but there was no check for this.

The result for non-triangular meshes varies, e.g.

>>> import pyvista as pv
>>> pv.Cube().decimate(0.5)
2022-07-03 17:48:56.522 (   6.404s) [        1C510740]vtkQuadricDecimation.cx:214    ERR| vtkQuadricDecimation (0x2b11330): Can only decimate triangles
ERROR:root:Can only decimate triangles
PolyData (0x7fde1bfa6dc0)
  N Cells:	0
  N Points:	0
  X Bounds:	1.000e+299, -1.000e+299
  Y Bounds:	1.000e+299, -1.000e+299
  Z Bounds:	1.000e+299, -1.000e+299
  N Arrays:	0

>>> pv.Cube().decimate_pro(0.5)
2022-07-03 17:49:09.501 (  19.383s) [        1C510740]     vtkDecimatePro.cxx:191    ERR| vtkDecimatePro (0x2b11330): DecimatePro does not accept polygons that are not triangles.
ERROR:root:DecimatePro does not accept polygons that are not triangles.
PolyData (0x7fddfd94fdc0)
  N Cells:	6
  N Points:	8
  X Bounds:	-5.000e-01, 5.000e-01
  Y Bounds:	-5.000e-01, 5.000e-01
  Z Bounds:	-5.000e-01, 5.000e-01
  N Arrays:	3

[codecov]

Codecov Report

Merging #2934 (051122a) into main (b12340a) will increase coverage by 0.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2934      +/-   ##
==========================================
+ Coverage   94.07%   94.28%   +0.20%     
==========================================
  Files          76       76              
  Lines       16534    16506      -28     
==========================================
+ Hits        15555    15562       +7     
+ Misses        979      944      -35