Feature: npz export for gustaf objects

[Roxana-P]

Gustaf already has great options for exporting meshes to nutils, mixd and with the help of meshio.
However, what if I want already have a gustaf mesh and just want to save it for later, or another script using gustaf?
This npz extension can save all kinds of gustaf entities - volumes, faces, edges and vertices. It saves not only the vertices and elements, but also the vertex data and boundary information. Thereby it is very suitable for saving meshes for interactive later or repetitive visualization.

Example:

import gustaf
import numpy as np

# Define coordinates
vertices = np.array(
    [
        [0.0, 0.0, 0.0],
        [1.0, 0.0, 0.0],
        [0.0, 1.0, 0.0],
        [1.0, 1.0, 0.0],
        [0.0, 0.0, 1.0],
        [1.0, 0.0, 1.0],
        [0.0, 1.0, 1.0],
        [1.0, 1.0, 1.0],
    ]
)

# Define hexa connectivity
hv = np.array([[0, 1, 3, 2, 4, 5, 7, 6]])

# Init hexa elements
hexa = gustaf.Volumes(
    vertices=vertices,
    volumes=hv,
)

# Create hexa elements and set vertex data
hexa.vertex_data['arange'] = np.arange(len(vertices))
hexa.vertex_data['quad_x'] = [v[0]**2 for v in vertices]
hexa.BC = {'min_z' : np.array([0, 1, 2, 3]),
'max_z' : np.array([4, 5, 6, 7])}

# Export hexa
gustaf.io.npz.export(hexa, 'hexa.npz')

load = gustaf.load('hexa.npz')

[j042]

thanks for this PR, could you add show_options?