Treat all the other (presumably newer) nonlinear cell types in UnstructuredGrid.linear_copy()

[tkoyama010]

Describe the feature you would like to be added.

In UnstructuredGrid.linear_copy() the docstring claims

Return a copy of the unstructured grid containing only linear cells.

But currently we have 10 QUADRATIC_* cell types and some more bi- and tri-quadratic ones, and only four of these is claimed to be converted by the method.

We should consider handling the linearization of the remaining nonlinear cell types, or explaining why some of these might not be applicable here (for instance the method considers quadratic quads and triangles in some way, but I'm not familiar with these cell types to know what's really going on in the method).

Originally posted by @adeak in #3115 (comment); edited here

Links to VTK Documentation, Examples, or Class Definitions.

pyvista/pyvista/core/pointset.py

Lines 1598 to 1620 in 5fe5661

	def linear_copy(self, deep=False):
	"""Return a copy of the unstructured grid containing only linear cells.
	Converts the following cell types to their linear equivalents.
	- ``VTK_QUADRATIC_TETRA --> VTK_TETRA``
	- ``VTK_QUADRATIC_PYRAMID --> VTK_PYRAMID``
	- ``VTK_QUADRATIC_WEDGE --> VTK_WEDGE``
	- ``VTK_QUADRATIC_HEXAHEDRON --> VTK_HEXAHEDRON``
	Parameters
	----------
	deep : bool
	When ``True``, makes a copy of the points array. Default
	``False``. Cells and cell types are always copied.
	Returns
	-------
	pyvista.UnstructuredGrid
	UnstructuredGrid containing only linear cells when
	``deep=False``.
	"""