First draft for general nspace function

[Bisaloo]

Fix #179

Breaking changes:

• dispace(): none

• trispace(): x, y

  • edit xlim and ylim in triplot()

• tcspace(): x, y, z, r.vec

  • edit achrbot in tetraplot()

[Bisaloo]

Note for the future if I don't manage to get this PR over the finish line now: we cannot really use this approach in the exisiting trispace() / triplot(), tcspace() etc. as they use a different reference simplex to convert the quantum catches to coordinates in the colour space.

In theory, it shouldn't matter as these values are arbitrary and any rotation, translation, etc. can be applied.

But:

• There is a long history of using these references and x, y, z might now have sense of their own because of this
• I'm not sure all users are aware that these values don't mean much in the absolute

[Bisaloo]

There is some back and forth in the commit history because I tried to already use it internally in trispace() / tcspace() but I think that was a distraction.

The function is already useful as it is so let's try to get this merged and we can always revisit & refactor later.