Delete CannotEvalException in v3: EvalNumerical and EvalBoolean return null where there is no value #1570

From #1543 (comment): in v3 a lack of value is null instead of an exception.

On master (a8a2b7e7) CannotEvalException is thrown by Entity.EvalNumerical() and Entity.EvalBoolean(), and the string extensions that call them, where the expression has no value, and by one precision check in the number operators (Operators.cs). The library catches it at one place, PartialFractions.HoldsAtSampledPoints, which #1497 replaces. #1553 moved the other probes to Evaled is Number.Complex.

In v3 the two return Complex? and Boolean?, and the exception type goes. See #1019.