Pynrm

- `get_avg_inbreeding` method that computes the average inbreeding rate of a given generation.
- `get_avg_ebv` method in `Pedigree` class that computes the average EBV of a given generation.
- Bug fix in `reproduce` method in `Simulation` class

- `Pedigree` class that contains generation number, sire id, dam id, estimated breeding value, and sex information for each animal. It uses `pandas.DataFrame` type to hold data.
- `Simulation` class that executes simulation of generational reproductive cycles. It has an instance variable of type `Pedigree` and updates it whenever a new generation is reproduced.
- `get_nrm` method that computes the value of numerator relationship matrix given a pedigree.