Allows for a color function to be specified to plot_field_lines()

[jeffreypaul15]

Fixes #50
@dstansby I do have one question about how we want to deal with the functionality, in #50 you mentioned that the color function would return a string but gave me an example with a function that returns rgba values as a tuple.
I've accounted for that as pyvista also does account for either a string or a list of rgb values.
However, the a from rgba or opacity isn't really accounted for.
I figured instead of doing

color.pop()
# we could do
opacity = color.pop()

and set the default opacity to 1.

I've also made a few changes to account for the passing of lists to field_arrays while saving and loading.

• Tests
• Example

[dstansby]

• Setting the default opacity to 1 sounds 👍
• This should have a test
• What's the status with saving the different colours? Could we always convert colours to RGBA values, and save them, instead of dealing with strings? Matplotlib has a function to do this here: https://matplotlib.org/stable/api/_as_gen/matplotlib.colors.to_rgba.html

[jeffreypaul15]

I agree that saving strings as rgba values makes more sense.
My question is do we have to worry about alpha or do we just set opacity as 1 for everything?

[dstansby]

If it's possible we should worry about alpha and set the opacity value appropriately.

[jeffreypaul15]

Yeah, I'll make that change

[jeffreypaul15]

Requires #73 to be merged

[jeffreypaul15]

@dstansby Do we need to add an example for this?

[dstansby]

Yes, it would be good to add an example. Just copying and modifying the current PFSS example to include a custom color function should be good.

[dstansby]

Suggested change

	# We cal also specify a color function while plotting the field lines.
	# We can also specify a color function while plotting the field lines.

[dstansby]

Could you also add a bit more description of what this function needs to look like? Something like

This function takes a single field line, and returns a color. In this case we use a Matplotlib norm and colormap to return a tuple of RGBA values.

[jeffreypaul15]

Sure thing!

[dstansby]

Is this a figure test? If not can we add a figure test with the custom color function?

[jeffreypaul15]

I was going to suggest that, do we have a new figure test for this one? Or modify the existing figure to use colorfunc instead of the default colours

[dstansby]

Lets modify the existing one

[jeffreypaul15]

Yep, I've done that