Replace custom clamp function with Base.clamp

[baggepinnen]

I think that Base.clamp should work nowadays. This would make it easier to spot clamp operations in equations (rather than max(min(...)) and replace them during linearization.

[ChrisRackauckas]

But is Base.clamp registered? If it isn't, it'll just trace to be the same thing, so not easier to see?

[baggepinnen]

It is

julia> @variables x
1-element Vector{Num}:
 x

julia> clamp(x, 0, 1)
clamp(x, 0, 1)