Only update camera state right before we need to

Originally reported by: Michael Zingale (Bitbucket: mzingale, GitHub: Unknown)

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consider the following script:

import yt
import numpy as np
from yt.visualization.volume_rendering.api import \
    Scene, \
    VolumeSource

# fake test dataset
ds = yt.testing.fake_vr_orientation_test_ds(128)

# no log please
ds._get_field_info("density").take_log = False

sc = Scene()

vol = VolumeSource(ds, field=('gas', 'density'))
sc.add_source(vol)


cam = sc.add_camera(ds, lens_type="perspective")
cam.resolution = (1280, 720)

cam.north_vector = np.array([0., 0., 1.])

cam.focus = ds.arr(np.array([1., 0., 0.]), 'code_length')

cam.position = ds.arr(np.array([0., 0., 0.]), 'code_length')


#sc.annotate_domain(ds)
sc.render()

sc.save("test_{:03}.png".format(0), sigma_clip=6.0)

this runs as-is.

If you change the order of the properties to:

cam.north_vector = np.array([0., 0., 1.])
cam.position = ds.arr(np.array([0., 0., 0.]), 'code_length')
cam.focus = ds.arr(np.array([1., 0., 0.]), 'code_length')

it aborts with

RuntimeError: Cannot set the camera focus and position to the same value

it you change them to

cam.focus = ds.arr(np.array([1., 0., 0.]), 'code_length')                                          
cam.position = ds.arr(np.array([0., 0., 0.]), 'code_length')                                       
cam.north_vector = np.array([0., 0., 1.])          

it aborts with:

yt.utilities.exceptions.YTException: normal_vector and north_vector cannot be aligned.

these checks should wait until the user is done setting all the properties and the order of setting them should not matter.

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• Bitbucket: https://bitbucket.org/yt_analysis/yt/issue/1299