3D scatter test

[ruaridhg]

Closes #8

• Added test_scatter_3D with brain_data
• Changed slice number to 9 so it was different from default to check test

Potential remaining tasks (@dstansby):

• Should I switch the default slice number for the 3D histogram and 3D slice tests?
• Not sure how to implement a 3D features scatter (is this necessary?)

[dstansby]

Should I switch the default slice number for the 3D histogram and 3D slice tests?

Do you think there's a benefit to doing this (e.g. is it testing something that wouldn't be tested if the slice isn't changed)? If so I'd say go for it!

Not sure how to implement a 3D features scatter (is this necessary?)

I think features data is always 2D, so we don't have to worry about 3D tests for that.

[dstansby]

• One comment to remove
• Isn't brain_data[0] an array already, so it shouldn't need casting to an array?