Handle properly m2ts fps

[moi15moi]

Currently, the TimestampsFactory.from_fps method always assume that the fps is perfectly constant.
In reality, with some format like m2ts, a "constant" fps isn't actually constant.

Currently, we do all the calculation in ms, but in ns, the equivalent would be: $ns = frame \times {1\over fps} \times 10^9$

But, for m2ts file, it is:

$$\begin{gather} ticks = {90000 \over fps} \\\ ns = \lfloor frame \times ticks \rfloor \times {1\over 90000} \times 10^9 \\\ ns = \lfloor frame \times {90000 \over fps} \rfloor \times {1\over 90000} \times 10^9 \end{gather}$$

For a example, see the end of the message here.

So, to properly handle this situation, we would need to add a time_scale parameter to from_fps: def from_fps(fps: Union[int, float, Fraction, Decimal], rounding_method: RoundingMethod, time_scale: Optional[int] = None) -> Timestamps.

But, by default, mkv round the timestamps to milliseconds, so I guess we may need another from_fps method where we could precise the timestamps precision (ex: ms) and if we round them or not.