Add length equation

Currently one cannot calculate length from the period.

Period is easier to measure (I think), users may want to get lengths this way.

We should add and function for that, e.g. by inverting expression for the period

```math
L = g (\frac{P}{2 \pi})^2

```

Start by locating the existing period calculation and any related tests. Use the equation in the issue as the specification for the new function, and consider the work done when users can calculate length from a supplied period and the behavior is covered by the relevant tests.

Written by the indexing model from the issue text.