Feat: implement round_series_to_precision() method and tests for it

[AlexTheWizardL]

This PR introduces the round_series_to_precision() method, which is necessary for implementing issue #35.

Key points:

• Python does not natively support method overloading, so instead of overloading the existing round_to_precision() method, I created a new method, round_series_to_precision(), for the vectorized version. This avoids adding type checks and splitting the logic within a single method.
• The tests are currently written under the TestLedger() class. These tests are expected to be refactored in milestone #2.

@lasuk Please review the changes and provide feedback or approval for merging

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[lasuk]

I have changed my opinion on the best way to combine the two related methods dealing with singleton and vector arguments.

Currently, we have one method taking singleton and another method taking pd.Series as arguments:

round_to_precision(self, amount: float, currency: str) -> float
round_series_to_precision(self, amount: pd.Series, currency: pd.Series) -> pd.Series

I propose instead to implement a single method where each argument can be either a singleton or a vector. The return value is a float if both arguments are singletons and a vector of float otherwise.

round_to_precision(self, amount: float | "vector of float", currency: str | "vector of str") -> float | vector of float

Motivation: The proposed solution is more flexible, it allows to provide one singleton and one vector as argument.

[lasuk]

Also, the method shall be open to any vector input in python, including arrays, rather than narrowly focusing on a pd.Series. This must be a quite common problem in Pyhton and we shall stick with the usual pattern used in python. We should also stick to the usual return type for such cases (array instead of pd.Series?).