Python library is missing operators and structs available in Rust

[ChristopherRabotin]

These need to be added after the __sub__ function in epoch.rs:

• __sub__ with an Epoch -- if it isn't possible to overload this function with pyo3, a specific function will need to be provided, e.g. timedelta to use as my_epoch.timedelta(another_epoch)
• Functions for the >, < operators
• __eq__ for Epoch and Duration

Duration also needs the operators >, <, ==. .