Tutorial 4 documentation suggestions

[rwbaer]

Some suggestions for improving tutorial 4 flow stemming from my misread of line 94 where we get an [expected] error trying to remove ellipse1:

It would be nice if the lead in read, "We can't remove a gate that is referenced by a Boolean gate. Therefore, we expect a GateTreeError if we try to remove 'ellipse1' which is referenced by BooleanGate 'ellipse_and_AposBpos'. Let's try it."

The error message reads, "GateTreeError: BooleanGate ellipse_and_AposBpos references gate ellipse1"

It would be nice if the error message were more like "GateTreeError: Can't remove a gate that is referenced by a Boolean gate. BooleanGate ellipse_and_AposBpos references gate ellipse1"

The second sentence alone is a little cryptic [unless you already understand in advance what is wrong]. The 2nd sentence alone might be understandable in the context of this tutorial but it will be less so in the wild.

Probable "typo" ??

In line 102 we discover that is_upper_corner.sum() is np.int64(25000). We then plot it, and you write, "Makes sense why the event count is 25%, it 1 of the 4 corners!" The highlighted picture is 50% of 25% which is 12.5%. The latter is indeed what is listed as the absolute_percent for upper_corner in res.report on line 100. Didn't you mean 12.5%?

I have not yet figured out how one submits pull requests, but in the future I might suggest such changes by that mechanism if that is okay.

Originally posted by @rwbaer in #260