Inconsistent return types & parameter naming

[berkay-yalin]

Inconsistent return types

Some of the distribution functions return a float whilst others return a decimal.Decimal.
All distribution functions should be consistent in their return types.

Inconsistent parameter naming

Inconsistent naming for parameters of various functions where the actual parameters represent the same thing.
For example, InvBinomialCD uses y whereas InvGeometricCD uses area:

https://github.com/berkay-yalin/python-probabilities/blob/d8585c2c41af3152a365e387ccb5b974b265aeca/python_probabilities/distributions_binomial.py#L59-L61

https://github.com/berkay-yalin/python-probabilities/blob/d8585c2c41af3152a365e387ccb5b974b265aeca/python_probabilities/distributions_geometric.py#L30-L35