simplify("(x^2)^0.5") should be "abs(x)"

While there are genuine use cases when returning "x" would be better (that's what most humans would expect), strictly speaking it is incorrect.

To reproduce in the mathnotepad:
```
derivative("(x^2)^0.5", "x").evaluate({x:-3})
1

derivative("(x^2)^0.5", "x", {simplify:false}).evaluate({x:-3})
-1
```
Here I evaluate the same derivative at the same point `x=-3` but get two different results.
Same problem for `x^4` and I assume all other even powers.