Refactor distribution to decimals

`get_distribution()` keeps a tally of how many times each face has been rolled, but some users are reporting that they want this same information as a percentage of total rolls.

Change this function to instead have the signature `vector GameDie::get_distribution()` that returns the *decimal* of rolls for each face relative to the number of *total* rolls. Each decimal should be a `double` between `0` and `1` inclusively. For example, if we have a 4-sided die that has rolled each face 1 time the `get_distribution()` function should return:
`{0.25,0.25,0.25,0.25}`

If there are no rolls yet, the function should report `0` for each face in the vector. Otherwise, the percentage should be calculated by **face rolls / total rolls**.