Add length equation (Menno)

**Current state**
The code does not include an equation to calculate the length of the pendulum.

**Proposed addition**
A function that calculates the length of the pendulum,$\ell$ based on the period, $T$:
$$\ell = \frac{g T^2}{4 \pi^2}$$

Inspect the Python repository for the existing pendulum calculations and identify the module or entry point where a length function belongs. Implement the stated period-based equation using the project's conventions, then add or run tests that verify the calculated length for representative periods.

Written by the indexing model from the issue text.