[Distributions] Add Binomial distribution & DiscretelyDistributed trait

[shivasankarka]

Traits

DiscretelyDistributed trait has been implemented with basic method for the distributions. Any integral value must be of type Int for consistency.

Binomial

• Binomial distribution has been implemented and it conforms to DiscretelyDistributed.
• New tests have been added for Binomial distribution.
• Updated imports & docstrings.

[shivasankarka]

@forfudan I have currently made the field n to be of Int type even though is must always be positive. Should we add these checks in the constructor and raise instead? (I didn't see any other raising functions in the library so I assumed that we are trying to avoid raising functions).

[forfudan]

@shivasankarka
Yes, at the moment I am trying to avoid raising. I think we can add it at a later time point to all distributions.

[forfudan]

Add checks for logpmf and cdf when k < 0.

For _log_binomial_coefficient(n: UInt, ...), change it to n: Int to be consistent with other cases. We may consider adding raises in future.