Improve DateObjectExpr with arithmetic methods

[axkr]

Improve DateObjectExpr with arithmetic methods plusYears, plusMonths, plusDays

See

• https://github.com/datahaki/tensor/blob/master/src/main/java/ch/alpine/tensor/qty/DateTime.java

[datahaki]

several continuous probability distributions can operate on DateObject's in combination with a parameter of type Quantity[1, "s"] (or any compatible unit) for instance

NormalDistribution, UniformDistribution, TrapezoidalDistribution, LaplaceDistribution, CauchyDistribution, LogisticDistribution, ...

see for example:

https://github.com/datahaki/tensor/blob/eb41eb5894b17ac60815bf6355b4c1470f996f64/src/test/java/ch/alpine/tensor/pdf/c/NormalDistributionTest.java#L159

[datahaki]

ups, Mathematica doesn't like it: