I implemented the Time_point class operators and functions, and the tests.
To implement the !=, <= and >= operators in the Time Point class, I decided to implement !=, <= and >= in the Duration class too, so that I could call them on the respective Time Point operators. I believe this makes the code more understandable, and they could be useful in case they are needed in the future.
For the Time Point tests, I created a test for every method and operator. I used EXPECT instead of ASSERT, so that the test continues in case some condition fails, so we can see if the other conditions also fail, and fix it easier.
For the Distant Future and Distant Past methods, I used std::numeric_limits, method infinity(). I used the tests to make sure no double value can be greater than that of distant future, or less than that of distant past.
I am not sure if the Julian Day method is correctly implemented. I simply converted the Julian days to seconds and added them as the duration. I don't know if an additional conversion is required.
I implemented the Time_point class operators and functions, and the tests.
Resolves #28 Resolves #18