Initial revision

* Modeled each molecule as a object
* Introduced `sweep()`
Take into account boundaries
Add command line argument to output result file
Parameterize box length
Molecule and iteration counts as user inputs
Rearrange function definitions
Better function name
Convert `as_tuple()` to a property
namedtuple awesomeness!
No need for option to write to file

If needed, just redirect in OS
Simplify probability distribution to molecular level
Correct feeling_lucky algorithm
Fixed bug with ZeroDivision check