Add extension feature for plotting fields when nodes are provided

[glwagner]

I noticed that there was code in the grids tutorial that could benefit from this feature.

[navidcy]

Like which code? I mean what's an example that benefits from what changed here so I can test it to approve?

[glwagner]

Like which code? I mean what's an example that benefits from what changed here so I can test it to approve?

This PR supports this kind of code:

c = Field{Center, Center, Nothing}(grid)
x, y, z = nodes(c)
heatmap(x, y, c)

The coordinates x, y can also be divided by a normalization (say 1000 to convert from m to km).

[navidcy]

Great! LGTM!

For future, we might wanna consider making this work:

c = CenterField(grid)
x, y, z = nodes(c)
heatmap(x, y, view(c, :, :, 1))

or

heatmap(x, z, view(c, :, 2, :))

[navidcy]

Great! LGTM!

For future, we might wanna consider making this work:

c = CenterField(grid)
x, y, z = nodes(c)
heatmap(x, y, view(c, :, :, 1))

or

heatmap(x, z, view(c, :, 2, :))

I take it back! What I suggested actually works!

[glwagner]

Great! LGTM!

For future, we might wanna consider making this work:

c = CenterField(grid)
x, y, z = nodes(c)
heatmap(x, y, view(c, :, :, 1))

or

heatmap(x, z, view(c, :, 2, :))

This PR supports this syntax I thought

[glwagner]

Ah yeah. I didn't put view in the example just to keep it simple but the point of all of this is to support all fields. view returns a Field so should be supported.