Removes awful hack for Discrete classes that affected every object.
Primitives and Operators have a normalPrec() analogon to bounds(),
and a computeNormalPrec(...) analogon to computeBounds(...).

By default, operators use the maximum of the normal precision of the
operands.

The discrete classes calculate their normal precision by first calling
bounds() to get their AABB after applying modifiers, then considering
their resolution and choosing an appropriate precision from this.

All other classes still use the default 0.0001 precision.