Add mvue and mom estimators of N

[jbytecode]

This PR implements two methods for estimating $N$ using a sample of integer IDs:

• Minimum-variance unbiased estimator (mvue)
• Methods of moments estimator (mom)

using a simple dispatcher function nestimate().

The problem of estimating $N$ has a famous and well-known use case, the German Tank Problem, is also referenced in the doc string of the method.

Basic tests are added.

Functions for confidence intervals are not yet implemented.

In my opinion, it would be nice to see these basic functions in StatsBase as they are the basics of their family of estimators.

I am so sorry If I am bothering the development team or doing something inconvenient.

Thank you in advance.