Added option to have an arbitrarily oriented cutting plane

[astrofrog]

This still needs some more testing, and need to think about best way to actually expose the parameters to the user. Example of adjusting the cutting plane interactively in Jupyter:

from ipywidgets import interact

@interact(a=(-1., 1.))
def update_a(a):
    current = list(vol.state.cutting_plane)
    current[0] = a
    vol.state.cutting_plane = current
    
@interact(b=(-1., 1.))
def update_b(b):
    current = list(vol.state.cutting_plane)
    current[1] = b
    vol.state.cutting_plane = current
    
@interact(c=(-1., 1.))
def update_c(c):
    current = list(vol.state.cutting_plane)
    current[2] = c
    vol.state.cutting_plane = current
    
@interact(d=(-256., 256.))
def update_d(d):
    current = list(vol.state.cutting_plane)
    current[3] = d
    vol.state.cutting_plane = current

Main TODOs:

• Have a toggle in the shaders for turning cutting plane on/off
• Implement UI options to set the cutting plane - could have simple mode where one just cuts in x, y, or z, and then have an advanced mode where all the coefficients can be edited.
• Added visual tests (either here or in glue-jupyter)