Fix issues with RectilinearGrid instantiation

[banesullivan]

These changes fix an issue where you couldn't update the coordinates of a RectilinearGrid after instantiating it.

The issue arose from how the dimensions have to be updated anytime the coordinates are changed. These changes make it so that the dimensions are automatically managed by PyVista and the user is unable to set them unless using the VTK method (.SetDimensions()). RectilinearGrids are technically implicitly defined, so it makes sense to only let the user update the x, y, and z coordinates for the mesh grid.

These changes also give users more options if wanting to create a 1D or 2D RectilinearGrid upon instantiation by allowing 1 or 2 coordinate arrays to be passed instead of 3. The __init__ method defaults to x first, then y, then z. If a user wants to have a 2D grid oriented on the yz plane, then they can set the coordinates after instantiating like so (this was previously not possible):

import pyvista as pv
import numpy as np
foo = pv.RectilinearGrid()
foo.y = np.array([3,4,5])
foo.z = np.array([6,7,8])
foo.plot(show_edges=True, show_bounds=True, lighting=False)

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This also adds a convenient cast_to_structured_grid() method as RectilinearGrids can easily be represented by a StructuredGrid.

Testing is implemented.

[banesullivan]

@pyvista/developers, anyone up for reviewing this?

[akaszynski]

Looks good to me!