Bounds on acos usages; fix a couple of links in readme

[jcao-bdai]

This is a first pass for the task "Bounds on acos and other functions with bounded domains", with focus on acos and asin usages.

• All usages of math.acos, math.asin reviewed;
• Changes included in this PR are cases with clear intention and the clampings added simply guard against floating point error (e.g. dot product of two unit quaternions should have norm <= 1).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a0bb12f) 76.47% compared to head (811ac8d) 77.04%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #108 +/- ## ========================================== + Coverage 76.47% 77.04% +0.56% ========================================== Files 24 24 Lines 5136 5136 ========================================== + Hits 3928 3957 +29 + Misses 1208 1179 -29 ```

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[petercorke]

I have a problem with the fix for log since s can be negative, so the s/|q| should be bounded in the range [-1, 1]. I think the other fixes are ok.