Adding isoluminant colour maps

[MBravoS]

I was not sure there was a strong case for this type of colour maps until I saw Figure 21 from Norris et al. (2021). We should look into this, but I would argue it has a lower priority compared to issues #2, #3 and #4. The interesting thing is that we can't use viscm for this, so we'll need to look to other methods.

[MBravoS]

Oh, I just tested setting both lightness ends to the same value on viscm and it actually works just fine, so doing this would be a lot easier than what I thought.

[MBravoS]

I've added two isoluminant colour maps with v0.8.0. They probably need more testing, and could always add more, but I think this is enough to close this issue.